Greeting from Bill Boxer, Sr. Vice President,
Apollo Sprayers International, Inc
Each month I get a chance to preview our guest writers article. Not only do I enjoy the opportunity but I also get to learn something new or reinforce a strong belief that I have on a topic. That is very much the case this month.
In my long history with spray finishing, HVLP technology and in communication with potential users of our products, I've learned that many people are not completely clear on what to expect from their spray finishing equipment and/or how to achieve the best possible results. I've always used one word in my writing and talks. Expectations. What are you looking to achieve straight off your application, be it brushing, wiping or spraying.
This month's article, by the extraordinary Greg Williams, is a must read toward understanding "expectations.” The more knowledge we have about process, the better finishers we become.
Birthday Special of the Month:
In honor and celebration of 50 years manufacturing HVLP Turbospray Technology,
Apollo Sprayers International, Inc. has manufactured a limited edition of the award winning
7500 Atomizer HVLPTurbospray Gun
To purchase the Gold Spraygun,
From the Apollo Archives:
We followed the A.100 Spray Gun with several new spray guns.
Now we are commemorating our 50th Anniversary with the precise,
Award Winning Gold 7500 Atomizer.
A. 100 spray gun
FEATURED ARTICLE BY Greg Williams, OFF-THE-GUN VS RUBBED FINISH
The Off-The-Gun Finish
No matter how good a spray technician you are, or how good the application equipment is, you are not going to apply a sprayed finish to a perfectly level “mirror finish”. Not yet.
Regardless of the coating, certain elements of chemistry and physics determine the smoothness of the coating as applied to a substrate.
The Mechanics of Spray Application
For evaporative coatings, that is, those which dry by evaporation of the solvents and other volatile elements of the coating, leaving the solid component on the surface of the sprayed object as a more or less consistent film, and for the evaporative/reactive coatings, which dry first by evaporation of the volatile components and then further cure by reaction of solids with oxygen or with a catalyst or drier these elements apply:
The liquid material is broken up into fine liquid particles which are propelled through the air to the surface of the object being coated. Energy is required to overcome the tendency of those particles to remain in a mass liquid form. The amount of energy needed varies according to how tenaciously those liquids stick together. Generally more viscous liquids require more energy to atomize. Think of honey versus water. Honey requires more energy to move it than water does, therefore it flows more slowly than water under the influence of gravity and would be more difficult to spray. The energy is supplied by air or fluid pressure, or both.
As the “shredded” liquid particles move through the air, they tend to take the shape that has the least surface area for the volume of the particle, that is, a sphere. With fast evaporating solvents such as acetone and alcohol, as used in many lacquers, the particles shrink in volume as they travel. As they lose the volatile element, they increase in viscosity. (Viscosity is resistance to flow, so a low viscosity is desirable for the atomizing and dispensing of the lacquer, but a higher viscosity is desirable to avoid runs and curtaining, especially on vertical surfaces).
When these liquid particles strike the surface, opposing forces encourage the liquid droplets to remain in a hemispherical drop on the surface (cohesion, surface tension), and to attach to the surface and to another droplet (adhesion, wetting).
If one drop comes into contact with another drop under the right circumstances, their surfaces will intersect and the two droplets will coalesce, or flow together. Again, under the right circumstances, such as the composition of the drops, the viscosity, and the amount of solvent available, many droplets will coalesce to form a more or less consistent film.
If the droplets are dried past a certain point, their surfaces will not coalesce, but will adhere to each other, and dry without leveling properly; leaving a surface that is somewhat bumpy and irregular. Other factors such as high ambient temperature, cold coating, airborne debris, and improper thinner can also cause an irregular or rough surface.
As the coating film continues to dry, it will continue to shrink. More irregularity can occur at this point.
All sprayed on finishes have, at some level, a characteristic pebbly surface. When the coating has dried and shrunken fully, but no further manipulation of the finish has occurred, we call this an “off-the-gun finish”. In many cases this is acceptable, especially if the surface is consistent and the irregularity is very finely textured. But we do see on many pieces of high end furniture, very level and consistent surfaces, many with quite high gloss and high distinctness of image (DOI), or a mirror like surface.
How does this happen?
The Rubbed Finish
Not to be confused with a rubbed on finish, the rubbed finish is one which, after drying and curing thoroughly, is abraded to level the coating and to establish the final sheen, or amount of gloss to the coating. Sheen is a function of the amount of light reflected off a surface in one direction from a source to a receiver (the eye, a camera lens, or a gloss meter).
These coatings are leveled with sandpaper that removes the surface irregularity and cuts a scratch pattern in the coating. This scratch pattern lowers the sheen of the surface by reflecting the light in all directions, (scattering) rather than primarily in one direction. The grit of the sandpaper determines the depth and width of the individual scratches, and also the sheen.
A coarser scratch pattern will provide a lower sheen, while a very fine scratch pattern will reflect more of the light in one direction, giving the appearance of a higher sheen. For the very high sheens, the scratch pattern will be invisible to the naked eye.
Both very high and very low sheen finishes present a maintenance problem. Very high sheens show even very small scratches, fingerprints, smudges, dust and dirt more than middle-level sheens. Very low sheens similarly show any oily fingerprints and dust, as well as shiny “rub” marks caused by even slight burnishing. High-gloss coatings magnify defects in the wood, as well as marks left by the sanding or staining process, and make any irregularities in the surface highly visible.
Surfaces in the 30- to 50-sheen range, (the metric is “gloss units”, as measured by a device designed for that purpose) are a bit easier to produce consistently and easier to maintain. A rubbed finish in this gloss range can be achieved by a process known in production work as “cutting and rubbing” or “wooling out.”
The basic procedure is to apply the coating in sufficient film thickness to allow abrading the coating with sandpaper and steel wool, allowing it to dry thoroughly, and then leveling it with the sandpaper. The leveled finish is then further abraded with a fine grade (4/0) steel wool to leave a finer and slightly different scratch pattern, thus raising the sheen.
Here’s a process for achieving a high satin or semi-gloss rubbed sheen on a lacquer coating:
After all of the sealing, filling and coloring steps are completed, apply several full wet coats of the lacquer, allowing each coat to dry thoroughly. This process may take several days, depending on how much coating has been applied, and the drying conditions. Make sure there is sufficient dry film thickness to allow the amount of sanding that will be necessary to make the coating quite level, especially on large, flat surfaces. Less topcoat will be needed on very small and curved surfaces, such as table edges, legs and stretchers.
Wet-sand flat areas like tabletops. The horizontal surfaces are generally more critical, but vertical panels, such as the doors of a wardrobe, are also important. The more visible they are, the more critical they are. Use P600 to P800 waterproof (often called wet or dry) sandpapers, wet with a mixture of Wool-Lube (a water miscible vegetable-based rubbing lubricant) and water. A mixture of 20 to 30 percent Wool-Lube to the water is generally a good choice, but as you sand with finer papers, a thinner mix might work better on some coatings.
Use a sanding block, which can be of cork or rubber, or a wooden block cushioned with a thin layer of felt, to keep the sandpaper flat and level on the surface. This allows the sandpaper to cut off the irregularities in the coating, resulting in a very level, low-sheen surface. Begin sanding with a finer paper; P800 is a good choice. If the coating is rough or very irregular, start with a coarser paper, such as P400. Switch to finer and finer grits when the scratches from the previous grit have been removed. Sand in any direction until using your finest grit, and then follow the grain.
The ideal would be to sand with the first paper until the coating becomes uniformly dull. Depending on the result desired, you can often stop when the coating is 60 to 80 percent dulled. This will have the effect of removing most dust nibs, overspray, orange peel and fatty edges, and leaving you with a very level surface.
Sanding with a block levels the surface, by only affecting the higher parts of the surface that the paper touches. For a perfectly level “mirror” or “piano finish,” sand with the first (coarsest) paper until the surface is uniformly dull. But for a typical satin, it isn’t necessary to sand that far, and you are less likely to cut through the coating by sanding less.
The less critical surfaces can often be left alone at this point or lightly sanded if necessary to remove dust, overspray, orange peel or other obvious surface irregularities. When you have leveled the surface to the desired degree, wipe off the wet residue prior to smoothing with 4/0 steel wool.
The steel wool should be liberally lubricated with full-strength liquid Wool-Lube, Murphy’s Oil Soap, flax soap, an emulsified oil furniture polish or a paste wax, depending on the look desired and whether a furniture polish or wax as a final step is intended. The first three lubricants can simply be wiped dry when the rubbing is done; the latter two buffed dry, leaving a thin film of oil or wax on the surface. If no film at all is desired, the first three can be wiped off with a damp cloth. Left on, they will provide a silky, smooth feel to the coating.
While petroleum distillate lubricants can be used, they have the disadvantage of being smelly, emitting harmful and flammable vapors, and requiring another solvent to remove any residual film from the rubbed surface.
The hardness and flexibility of the coating can affect how well — and how easily — that coating may be rubbed or polished. A very hard and brittle coating may be rubbed to a high shine with much effort, while a somewhat softer and more flexible coating may take much less effort, but might not take as high a gloss. A very tough and flexible coating such as a spar varnish will be difficult to rub out and may not take a consistent sheen. And very soft finishes such as drying and semi-drying oils (Tung oil, linseed oil, Danish oil) do not rub well at all.
As the finish wears, developing scratches, dull spots or higher-sheen burnished spots, the final steps of the rubbing process (steel wool and lubricant) can be repeated, bringing back the clean, smooth and even look.
Few things will so improve the look and feel of a film coating on wood as rubbing it out. The satin/semi-gloss sheen as described here is easy to live with and attractive on most types of wood furniture. And it’s easy to perform with a little practice.
Q & A from Our Readers
Q: I need to finish the insides of some drawers. I am worried about the finish off gassing.
A: Use a water-based finish of shellac. The solvents in these coatings evaporate slowly so they don’t leave a smell. Let the finish dry and smell it.
When it doesn’t smell any longer, it is ready to go.
Q: I still want to use finishes with volatile solvents. Unfortunately, my only finishing space right now is a basement.
A: You must have a space with absolutely no risk of fire or sparks. Lights, motors and switches must be non-sparking and fireproof. Volatile vapors can quickly fill a room, often sinking towards the floor when they’re heavier than air, and then ignite in a fireball from something so simple as you pulling an electrical plug out of the wall.
Your basement is filled with switches and outlets, and your furnace and hot water heater are like detonators just waiting to go off.
Take a Break and Have Some Fun!
See how many words you can find. Look for words horizontally and vertically. You can print this page, including the puzzle, work offline and then highlight words as you find them.