Preparing Wood for a Finish

The primary reason you need to sand wood is to remove the washboarding and other mill marks caused by machine tools. On this board, the washboarding, which was caused by a planer and has been highlighted with stain, is particularly severe. It would be most efficient to begin sanding with #80-grit sandpaper.

The reason you have to sand wood before applying a finish is to remove machine marks. All machine tools leave cuts or impressions in wood that are highlighted by stains and finishes, especially by stains. Before machine tools appeared in the mid-nineteenth century no sanding was needed. Indeed, there was no sandpaper. Wood was smoothed with hand planes and scrapers.

You can still use hand planes and scrapers to smooth wood; you don’t have to use sandpaper. You can hand plane or scrape the wood straight from the saw, or you can begin the smoothing with a jointer and planer and then finish off with a hand plane or scraper. You can also use molding planes and scratch stocks to shape wood rather than routers and shapers.

But few woodworkers choose this route because machine tools are much faster and easier to learn to use than hand tools. The price, then, for using machine tools alone for smoothing and shaping wood is that you have to finish off with sandpaper to take out the machine marks.

This is an important psychological point. It makes sanding less burdensome when you remind yourself that you don’t have to do it. It’s just the price you pay for the increased efficiency of substituting machine tools for hand tools.

How to Sand

The trick to efficient sanding is to begin with a sandpaper grit that cuts through machine marks and other problems in the wood with the least amount of effort and without creating larger-than-necessary scratches that then have to be sanded out. This holds true whether you are sanding by hand or using a sanding machine.

When you have removed all the machine marks with the coarsest grit you choose, sand out the scratches left by this sandpaper using increasingly finer-grit sandpapers until you reach the sanding grit that produces the size scratches you want.

As an aid to determining when you have sanded out all the machine marks, draw some marks on the wood using a no. 2 pencil and sand until no evidence of these marks remain. To be extra sure you have sanded enough, do it again.

Random-orbit sanders are more efficient than vibrator sanders, but they still leave tiny swirl marks in the wood. The best policy is to sand them out by hand in the direction of the grain after sanding to the finest grit, usually #180 or #220, with the sander. Doing this is especially important if you are staining.

In practice, the best grit to start with is usually #80 or #100 grit. But, if the problems in the wood are so severe that #80 grit doesn’t remove them quickly, drop back to a grit that does.

On the other hand, if the problems can be removed with a finer-grit sandpaper, such as #120 grit or #150 grit, you are wasting time and energy if you begin sanding with a coarser-grit sandpaper. For example, factory pre-sanded veneered plywood or MDF is almost ready to use. Sometimes, beginning with #150 grit is as coarse as you need. Rarely do you need to begin sanding with coarser than #120 grit. Also if you’re refinishing, you can usually sand with just #180 grit to be sure you’ve removed all the previous finish. The wood was sanded originally, after all.

The best grit to end with is usually #180 or #220 grit, though it can be helpful to sand to #320 or #400 grit to reduce grain raising when using water-based finishes. Woodworkers disagree about which grit to sand to. I rarely sand beyond #180 grit.

The primary goal of sanding is to produce a surface that doesn’t show either machine marks or sanding scratches after you apply a stain or finish. If the scratch pattern can be made even, you may achieve satisfactory results sanding only to #150 grit. Stationary sanding machines are best for doing this, though hand sanding will also work, especially if you aren’t applying a stain.

If you could sand just the right amount with each sandpaper grit, it would be most efficient to go through each consecutive grit—#80, #100, #120, #150, #180—and so on. But most of us sand more than necessary with each grit, so you may actually spend less effort skipping grits. This is especially the case when using machine sanding tools.

But sanding is very personal. We apply different pressures, use sandpapers to different degrees of wear, and sand for varying lengths of time. The only way to know for sure that you have sanded enough is to apply a stain and see if any machine marks or sanding scratches show. It is therefore wise to practice on some scrap wood until you get a feel for what works best for you.

The most efficient use of sandpaper when backing it with just your hand is to tear the sheet into thirds crossways and then fold one of the thirds into thirds lengthways. Flip the thirds to use 100 percent of the paper.

If you are sanding a flat surface such as a tabletop, and you want to keep it flat, always back your sandpaper with a flat sanding block. Ideally, the block should be soft enough to serve the dual purpose of absorbing coarser-grit dirt that may get between the block and the sandpaper, or between the sandpaper and the wood. There are good sanding blocks of all sizes and shapes on the market, including here. You can also make your own sanding block by gluing gasket cork to the bottom of a wood block.

Sanding with your fingers backing the sandpaper will cut away softer spring-growth wood faster than harder summer-growth wood. This will leave ridges in coarse-grained woods such as oak that you may not notice until after you have applied the finish.

Always sand in the direction of the wood grain when possible. Sanding across the grain or diagonal to it tears the wood fibers, leaving scratches deeper and more obvious than the scratches left with the grain.

Random-Orbit Sanders

Most woodworkers use random-orbit sanders because they are efficient, easy to use, and they leave a less-visible scratch pattern than vibrator sanders due to the randomness of their movement. For both of these sanders, however, there are two critical rules to follow.

First, don’t press down on the sander when sanding. Let the sander’s weight do the work. Pressing leaves deeper and more obvious swirl marks, or “squigglies,” that then have to be sanded out. Simply move the sander slowly over the surface of the wood in some pattern that covers all areas approximately equally.

Random-orbit sanders are easy to use and efficient for smoothing wood. To reduce the depth of the swirl marks, use a light touch. Don’t press down on the sander. Let its weight do the work .

Second, it’s always the best policy to sand out the squigglies by hand after you have progressed to your final sanding grit (for example, #180 or #220), especially if you are applying a stain. Use a flat block to back the sandpaper if you are sanding a flat surface. It’s most efficient to use the same grit sandpaper you used for your last machine sanding, but you can use one grit finer if you sand a little longer.

You can also use a belt sander, but it will be difficult to control. The slightest rocking motion will leave gouges in the wood that will take a lot of work to sand out.

Removing Sanding Dust

No matter whether you sand by hand or with a machine, always remove the sanding dust before advancing to the next-finer grit sandpaper. The best tool to use is a vacuum because it is the cleanest. A brush kicks the dust up in the air to dirty your shop and possibly land back on your work during finishing.

Tack rags load up too quickly with the large amount of dust created at the wood level. These sticky rags should be reserved for removing the small amounts of dust after sanding between coats of finish—except with water-based finishes, when you should use a water-damp cloth instead.

Compressed air works well for removing sanding dust if you have a good exhaust system, such as a spray booth, to suck the dust out of your shop.

It’s not necessary to be overly compulsive about getting all the dust out of the pores. Vacuums, brushes and compressed air all do an adequate job. You won’t see remnants of dust deep in the pores under a finish, or under a stain and finish. Just get the wood clean enough so you don’t feel or pick up any dust when wiping over the surface with your hand.

In fact, just before applying a finish, whether directly to the wood or over a previous coat, wipe your hand over the surface to check that it is clean and to remove any small amount of dust that may have settled. Clean your hand by wiping it on your pants leg.

The Role of Flatting Agent in Creating Sheen

The sheen of a finish is the degree of light reflection when a surface is viewed at a low angle. In a high sheen or high gloss surface, you see glare or a distinct reflected image. In a low sheen, satin or flat surface, glare and reflection are softened to the point of non-existence.

Flatted finishes (left) produce little or no reflection while gloss finishes with no flatting agent added (right) reflect glare and image

Oil finishes always produce a softened satin sheen. Wax finishes produce a little more shine than oil. All film-building finishes—varnish, lacquer, water-based finish and shellac—can be made to have any sheen, from high gloss to very flat.

There are two ways to produce a flatted sheen: rub the finish with abrasives of various grits to produce the sheen you want, or use a finish with the sheen already built in by the inclusion of a “flatting agent.” Varnish (including polyurethane varnish), lacquer and water-based finish are available in several sheens. Shellac is supplied only in gloss.

Both methods of creating a flatted sheen—rubbing with abrasives and using a product containing flatting agent—work by creating a microscopically rough surface that reflects light randomly.

This article is about the role of flatting agent and how it works.

Flatting agent

Flatting agent is the solid stuff that settles to the bottom of a can of clear finish and has to be stirred into suspension before use. If a finish doesn’t contain a flatting agent, the finish will be glossy when it dries.

The flatting agent is composed of small particles of solid material, usually amorphous silica, the nature of which makes the particles invisible and transparent in the cured finish film. As the wet finish shrinks during drying, it pulls taut over the particles near the surface, and this creates the micro-roughness that gives the flatted effect.

The particles aren’t exposed; they are shrink wrapped by the finish film. You see the shrink wrapping occurring as you watch the flatted finish dry. At first, when you apply the finish, the film is glossy. But as the solvent or thinner reaches a certain point of evaporation, the flatting appears rather suddenly.

In most cases you buy the finish with the flatting agent already added. But flatting agent, usually called “flatting paste,” is available separately from suppliers to the professional finishing trade for you to add yourself.

Most manufacturers of flatted finishes use loosely defined terms, such as semi-gloss, satin, eggshell, flat and matte, to describe what they are selling you. Some manufacturers who sell to professional finishers use a numbering system that runs from 90 (high gloss) down to 10 (very flat). This is the more accurate and helpful system and should be used by everyone.

Few manufacturers targeting non-professionals offer more than two or three choices of sheen. Most sell only gloss and satin. Even with just these two varieties, however, you still have total control over the sheen you will get simply by pouring off some of the gloss from a can of satin in which the flatting agent has settled and mixing the two parts.

Add more of the poured-off gloss to get an in-between sheen. Add more of the remaining flatted part to get a flatter sheen than you began with.

Be sure to keep the store clerk from shaking the can, or let the can sit undisturbed on your shelf for several weeks before pouring off some of the gloss.

Because the flatted effect is created by a roughened surface, it should be obvious that a flatted sheen can be made glossy in the same way that a rubbed satin or flat sheen can. Simply level the surface with fine sandpaper, then polish with very fine abrasives.

Likewise, a gloss sheen can be made satin or flat by rubbing with abrasives that produce the sheen you want.

In other words, you have total control over the sheen you end up with.

Myths

There are two common myths about flatting agent that are often repeated among professional finishers and store clerks.

The first is that the flatting effect is cumulative either because each coat adds to the flatness of the previous coat, or, in the case of lacquer, because all the flatting agent from each coat floats to the top of the last coat applied.

The second is that flatting agent weakens the scratch resistance of the finish film.

Cumulative Effect

Because the flatted effect is created not by the particles embedded deep within each coat of finish film, but only by those particles located near the surface, there is no cumulative effect caused by applying more coats. The only way to increase the flatness of a finish is to add more flatting agent to it. You can continue to apply coat after coat of any given flatness and the resulting sheen will not change. It’s the last coat applied that establishes the sheen.

It’s the last coat of finish applied that determines the sheen. This is true with all finishes. The left quarter of this panel has three coats of gloss polyurethane varnish applied. The second quarter has three coats of gloss followed by one coat of satin. The third quarter has three coats of satin polyurethane varnish followed by one coat of gloss. The right quarter has three coats of satin. There’s no difference in the gloss of the first and third quarters even though the third quarter has three coats of satin underneath. And there’s no difference in the satin of the second and fourth quarters even though the second has three coats of gloss underneath.

You can easily confirm this by applying a gloss coat over a satin or flat coat. When the finish has dried, the surface will be glossy just as if all the coats had been gloss.

Nor does flatting agent rise to the top of a coat of finish. If anything the flatting agent settles, just as it does in a can. At any rate, flatting agent surely doesn’t transfer from one coat to another.

This myth may have gotten started because finishers have noticed that a flatted finish can become glossy after years of use. What has really happened, of course, is that the micro-roughness on the surface has been leveled and polished by wear, and the remaining flatting agent in the film is transparent.

Scratch resistance

Many flatted finishes appear to scratch more easily than gloss finishes (though a flatted finish doesn’t show scratches as clearly as a gloss finish). But because the particles at the surface are all covered (shrink wrapped) with the finish, there can’t be any difference in scratch resistance.

There can, however, be the appearance of less scratch resistance because it’s easier for coarse objects to level the micro-bumps than to dig into a level film.

To aid in resisting micro-bump leveling, many manufacturers use wax-coated, amorphous silica, but there’s no way for you to know in advance because this information is never provided.

The wax-coated particles serve an additional purpose of keeping the flatting agent from clumping at the bottom of the can and also from clumping around the lip of the can after you have poured some finish from it. Clumping, if it does occur, shows up as tiny white specks in the finish and is impossible to redissolve. You’ll have to switch to a fresh can of finish, in which case I would suggest changing brands.

TIP: Wipe Stain, Don’t Brush

The rule for applying stain evenly is to apply a wet coat and wipe off the excess before the stain dries. With faster drying stains such as water-based stain, dye stain and lacquer stain, it’s not possible to get the stain applied fast enough on large surfaces if you use a brush to apply the stain. You should either spray the stain or wipe it on using a wet cloth.

Wiping Stain

It’s also best to spray or wipe with slow drying oil stain because it doesn’t make sense to waste all that time brushing.

Spraying is fast and efficient, but cleaning the spray gun isn’t, especially if you have to get it clean enough to spray a clear finish on unstained wood.

So, unless you have a spray gun dedicated to staining, it’s most efficient to wipe on the stain. As long as the cloth is thoroughly wet, you can easily get the stain into inside corners, grooves and recesses. But if you are having difficulty, you can always make use of an inexpensive throwaway brush to work the stain into these areas.

TIP: Testing for Fish Eye

Fish eye is a term that describes the ridging or cratering of a newly applied finish; the finish refuses to lay down flat. Fish eye is common during refinishing because it is caused by the low-surface-tension silicone oil contained in many furniture polishes getting through the deteriorated original finish and into the wood.

Test for Fish Eye

There are two easy ways to test stripped wood to determine if you may have a fish-eye problem. If you are staining the wood, apply a wet coat of stain. If you aren’t staining, apply a wet coat of mineral spirits if you intend to use a solvent finish, or a wet coat of water if you’re going to use a water-based finish.

If there is silicone contamination in the wood, the stain, mineral spirits or water will bunch up into ridges or craters wherever there is silicone, and you will know that you need to take precautions against the contamination.

TIP: Brush Efficiently

When brushing a finish onto a large horizontal surface such as a tabletop, it’s most efficient to lift a brush load of the finish (the bristles dipped about halfway into the finish) out of the container and plop it down at the center of the area you want to brush. Then stretch out that puddle of finish from end to end working in the direction of the grain.

Airplane Landing

Work fast without dragging the brush over the edge at each end, which would result in runs down the side. You can accomplish this by using airplane-like landings just in from each edge as you brush back and forth.

When you have lined up the brush strokes end to end at least one brush width wide, plop down another brush load about an inch from the previous strokes and stretch it out end to end while working it back into the existing finish. Continue with this procedure until you have covered the surface all the way across.

To avoid drips where you don’t want them, hold the container of finish in your other hand near where you deposit the brush load of finish.

Seven Types of Stain

We use the term “stain” to identify a colorant we apply to wood to change its color. But stains are not equal. Besides the obvious differences in color, there are at least seven categories of commercial stains that each apply and color differently. If you really want to have control over staining, you need to understand the differences and how to identify and choose each type.

Oil Stain

Oil stains are the most widely available and the type of stain most people think of when they think of stain. These are the easiest to use because the linseed oil (sometimes a mixture of linseed oil and varnish) binder allows plenty of time to remove the excess before the stain dries—even on large projects.

You can identify oil stains by their thinning and clean-up solvent: mineral spirits (paint thinner). Most manufacturers list it as “petroleum distillate.” Some brands use the more technical (and user unfriendly) name: “aliphatic hydrocarbon.”

Though some oil stains contain only pigment, many contain pigment and dye and some contain only dye. The type of colorant used doesn’t seem to make a lot of difference in the way the stain looks on the wood, however, because of the impact of the binder. (This goes for all stains with a binder included.)

Choose an oil stain to apply under any finish except water base, and in all cases where you don’t need any of the special characteristics offered by other stains. Allow overnight drying in a warm room before applying a finish.

Varnish Stain

Varnish stains resemble oil stains in every way but one. Varnish stains use only varnish (sometimes polyurethane varnish) as the binder, so varnish stains dry hard while oil stains don’t. Therefore, a varnish stain can be brushed on wood and left to dry without wiping while excess oil stain has to be wiped off or the finish applied on top may chip or peel.

Think of a varnish stain as alkyd paint with less colorant added.

Fortunately, most manufacturers label their varnish stains to distinguish them from oil stains because varnish stains use the same thinner as oil stains: mineral spirits. If you aren’t sure whether a stain is varnish or oil, put a puddle of stain on top of the can or on another non-porous surface and see if it dries hard after several days in a warm room. Thick oil stains never harden.

Varnish stains are more difficult to use than oil stains because there’s less time to wipe off excess. Brushing and leaving the excess usually leaves prominent brush marks that stand out because they’re colored.

Choose a varnish stain to overcoat an already stained and finished surface that is dull or scuffed, or if you’re wiping off excess on a small project.

Water-Based Stain

Water-based stains use water-based finish as the binder and replace most of the organic thinner with water. So these stains pollute less, are less irritating to be around, and are easier to clean up than oil or varnish stains.

You can identify water-based stains by their thinning and clean-up solvent, which is water.

Water-based stains are usually the best stain to use under water-based finishes because these finishes don’t bond well over oil or varnish stains unless you give them a week or longer to thoroughly dry. Unfortunately, water-based stains are more difficult to use because they raise the grain of the wood, and they dry fast.

Sanding off raised grain inevitably leads to sanding through color in places. To avoid this, raise the grain and sand it off before applying the stain, or bury the raised grain.

To raise the grain first, wet the wood with a wet cloth. Let the wood dry overnight or at least for a few hours in a warm, dry room. Then sand off the roughness and apply the stain. To bury raised grain, simply apply the first coat of finish over the stain and raised grain, then sand smooth.

Overcoming fast drying is more difficult. You can add a slow evaporating solvent (usually propylene glycol) provided by some manufacturers, or you can add lacquer retarder. But adding either reduces the color intensity of the stain and defeats the main purpose of using water-based products in the first place—to reduce exposure to solvents.

A better method is to divide your project into smaller parts and apply and wipe off the stain on each before going to the next. You can also have a second person follow you, quickly wiping off the excess.

Choose a water-based stain for use under a water-based finish.

Gel Stain

Most gel stains are oil-based, so they thin and clean up with mineral spirits. They are identifiable by their thickness, which is similar to mayonnaise. This makes them rather messy to apply, but gel stains solve the single biggest problem in wood finishing—blotching on pine.

Blotching is uneven coloring caused by varying densities and resin deposits in the wood and is the only problem that can’t be fixed by stripping and starting over. The only ways to remove blotching are to sand it out, which is very time consuming, or paint the wood, which is seldom a desired solution.

So gel stains serve a very important role in wood finishing. And they are much more predictable and easy to use (only one product to apply) than applying a wood conditioner or washcoat before staining, which is the method most often suggested.

Choose a gel stain especially when staining pine, but also when staining other blotch-prone woods if you want to reduce the blotching.

Lacquer Stain

Lacquer stains use very fast drying binders and solvents. Professional finishers love these stains because the finish can be applied within 15 minutes or so, and the stain can be added to lacquer to make a “toner” for adjusting color between coats of finish. It’s probably this use that has given these stains their name because they don’t use lacquer as the binder. They use a very fast drying varnish—a short-oil varnish.

You can identify lacquer stains by their strong, pungent odor caused by the solvents, which usually include xylene and various ketones. These should be listed on the cans.

Lacquer stains are difficult to use because of their very fast drying. Professionals usually work in pairs, with one person spraying the stain and the other following right behind wiping off the excess.

Choose a lacquer stain if you want to reduce the time between staining and finishing and are working together with someone else or are staining small surfaces. Also choose a lacquer stain if you want to add a colorant to lacquer.

Water-Soluble Dye Stain

Water-soluble dyes are sold in powder form, which makes them easy to identify. These dyes are also referred to as “aniline” dyes and were developed in the late nineteenth century for use on textiles and then adapted for wood. They were very popular in the furniture industry until the 1950s when metal-complex dyes were developed, and they continue to be popular with amateur and small-shop woodworkers because of their richness, wide choice of colors and ease of use.

To make a liquid dye from the powder, simply dissolve it in water. The ratio of one ounce of powder in one quart of water usually makes a standard color, but you can dissolve more or less powder to create a more or less intense color. The hotter the water the more powder you can dissolve. Use distilled water if there’s so much metal residue in your tap water that it affects the color.

The dye has an infinite shelf life in both powder and liquid forms.

Water-soluble dyes have two great advantages and one disadvantage when compared to the stains above.

One advantage is that the dye doesn’t obscure the wood no matter how dark you get it. A black dye, for example, can ebonize wood without completely hiding the figure.

Also, because there’s no binder in the dye, you can darken or lighten or change the color right on the wood even after the dye has totally dried. Apply more dye to darken the color. Wipe with a wet or damp cloth to lighten the color. Apply a different colored dye to change the color.

The problem with dye is that it fades in UV light, which means sunlight and fluorescent light. You may not want to use a dye if your project will be subjected to one of these light sources.

Two related but more seldom used dyes are alcohol-soluble and oil-soluble powder dyes. Alcohol-soluble dyes are sometimes used by touch-up specialists in combination with shellac to take advantage of the fast drying characteristic, and oil-soluble dyes are sometimes added to oil stains.

Choose a water-soluble dye if you want deeper or more even coloring than can be achieved with stains that contain a binder, or you want more control of the color after you apply the stain to the wood.

Metal-Complex (Metalized) Dye Stain

A weakness of dye is that it fades fairly quickly in UV light, so a more fade-resistant dye was developed in the 1950s called “metal-complex,” or “metalized” dye. This dye still fades, just not as rapidly.

Metalized dyes are usually available thinned with acetone (methanol was once used) and ready to use. These dyes are labeled “non-grain-raising” or “NGR” and are very popular in industry and in shops that spray their stains. The dye can be sprayed directly on the wood to act as a stain, and it dries very rapidly so finish can be applied within minutes. The dye can also be added to lacquer and sprayed as a toner.

Metalized dyes are also available in concentrated liquid form for you to thin with water, alcohol, acetone or lacquer thinner. Using water provides more time for application, of course, but introduces grain raising.

Choose a metalized dye stain if you want a deeper or more even coloring than can be achieved with pigment. Choose also if you want to reduce the time between staining and finishing or add a dye colorant to lacquer.

TIP: Boiled Linseed Oil isn’t Boiled

Boiled linseed oil isn’t raw linseed oil that has been boiled. You can’t make boiled linseed oil simply by heating it.

Boiled linseed oil, which is sometimes abbreviated “BLO,” is raw linseed oil with metallic driers added. These driers act as catalysts to speed the introduction of oxygen into the oil, which causes the crosslinking and thus the curing to go faster. So boiled linseed oil dries or cures much faster than raw linseed oil.

In fact, raw linseed oil cures so slowly (it takes weeks even with all the excess wiped off the wood) that there’s really no legitimate use that I know of for raw linseed oil in interior wood finishing. Boiled linseed oil cures overnight in a warm room with the excess wiped off.

The term “boiled” probably comes from a previous time when the linseed oil was heated to incorporate the lead driers. Lead is no longer used as a drier in common linseed oils, only in some artist’s and other specialty oil paints. But the term “boiled” continues to be used.

Instead of lead, manufacturers now use synthetic driers that come in liquid form. An example of a widely available consumer drier is “Japan” drier.

TIP: Metamarism

The light source makes a big difference for how your finished wood projects appear. It’s important to be aware of this if you are working on a project in your shop under one light source and then moving the project to another light source.

Look at the four accompanying photos of stained mahogany. I set the white balance on my digital camera for daylight and took the first picture outdoors in the shade. Then, without changing the white balance, I took additional pictures indoors under the cool-white fluorescent lighting I use in my shop, the full-spectrum fluorescent lighting I use in my finishing area, and finally the incandescent lighting in my house.

Notice how the cool-white lighting brings out more green in the stained wood, the full-spectrum lighting produces a color that is closest to natural daylight and the incandescent lighting brings out more red tones. This is what you should expect when you move furniture from one light source to another. The phenomenon is called “metamerism.”

The Same Wood Finish Under Different Lighting

TIP: Shellac Thinner

The thinner and clear-up solvent you should use with shellac is denatured alcohol. This is ethanol, the same alcohol that is included in beer, wine and liquors. But it is made poisonous so it can be sold without liquor taxes.

Sometimes you’ll see methanol (methyl alcohol) sold in paint stores. Methanol works fine for thinning shellac, but it is quite toxic if you are around it for a long time breathing the vapors. So it isn’t a good idea to use methanol unless you are working with a good exhaust.

Isopropyl rubbing alcohol is no good for thinning shellac because it contains about 30% water. The water will cause the shellac to turn white, or “blush.” If you have access to 95-to-100 percent pure propanol or isopropyl alcohol, you could use it for thinning without a problem.

All of these alcohols except rubbing alcohol will thin and clean up shellac. The difference among them is evaporation rate. Methanol evaporates the quickest. Denatured alcohol is next. And propanol and isopropyl alcohol are the slowest.

TIP: Test for Good Bond

Poor Bond

There are a couple of good tests you can perform to see if a finish you are using bonds well to whatever is underneath—oily wood, stain, filler glaze or whatever.

The better test is called the “cross-hatch” test. Using a razor blade or box cutter score the finish (on scrap wood, of course) in a cross-hatch pattern: half-a-dozen cuts about a millimeter apart running in one direction and half-a-dozen cuts in the perpendicular direction.

Then press masking or Scotch tape onto the cut pattern and lift it quickly. If the cuts remain fairly clean, you have a good bond. If the cut lines are jagged, the bond isn’t as good. If some of the tiny squares transfer to the tape, you have a very poor bond.

The picture shows a very poor bond.

The second testing method is quicker but may require some experience to interpret accurately. Take a coin and draw it hard across an inch or two of the finish. Does it merely dent the finish (a good bond), or does it tear up and remove the finish (a poor bond)?