Back in my auto body days we had what we referred to as a D. A. Sander. This stood for dual action. Today in the woodworking world we know them as random orbit sanders. I knew these worked extremely well on wood and used one in my workshop. I was glad to see them evolve into the woodworking industry. The random orbit sander has probably done more to cure sanding issues than any other single item I know of including increasing the speed of getting a sanding job done.
There are many sanders in various configurations and today they almost all use the random orbit technology. This is where the pad semi-oscillates in a random scratch pattern that is not discernible to the eye. How much scratch is seen depends on the grit and the stroke length, meaning sanders that have more of a vibrating action than a true oscillating action can leave visible scratches.
Photo 1: In this example we see a vibrating scratch pattern made with a mouse or a jitterbug sander.
Photo 2: In this example we see a fast, oscillating scratch or better yet, we don’t.
Photo 3: In this photo we see an oscillating scratch that was produced with a random orbit sander. The sanding scratches are seen because we used aggressive 120 grit on a soft wood (pine). The sander was allowed to build up speed prior to engaging and left the circle marks. Always start with the sander engaging the wood. While all of the sanders do a good job, be aware of the scratch pattern your sander makes. It is a good idea to test it with your planned finish. Using finer grit sandpaper will usually eliminate the scratches. Caution, never allow your sander to gain speed before engaging the wood, this can cause serious scratches and dips in the wood.
Photo 4: Example of inline scratches from a drum sander
The results from a drum sander may look pretty good; however, I have never found a drum sander or wide-belt sander to create a surface ready for finishing. The board in Photo 4 was sanded with 150 grit sandpaper on a drum sander. We can see the in-line scratches left behind. This will require additional sanding with a hand sander before finishing.
It is very important that your entire piece be final sanded with the same grit sandpaper or you can have a color deviation. In this Photo 5 you can see that we took a piece of poplar and sanded sections with different grit sandpapers. I then applied one coat of the dye across the board. Note the color differences across the board. With this knowledge we can use it to our advantage.
Photos 6 & 7: We started sanding with 120 grit sandpaper. We then progressed through the grits and finally sanded with 600 grit sandpaper. Note how much lighter the same dye application appears when comparing the two photos.
The finer the grit of sandpaper used, the lighter the color because the wood becomes burnished and does not absorb the color as much as it does if sanded with a coarser grit.
• I cannot stress enough the importance of pre-sanding intersecting parts and thinking ahead prior to assembly. Pre-Sanding and trace coating will improve the quality of your finish and your project.
• While the lesser expensive sanders do work, the high-end detail sanders for close quarters, corners, etc. are much superior.
• I find that cheap sandpaper doesn't hold up and can often have inconsistent grits that can create issues.
• When you have completed the sanding, make a quick check for anything missed by wiping your project with either naphtha or mineral spirits to wet it and pay close attention, you can often detect anything you may have missed.
• Only in special cases do I sand my furniture with a finer grit that 180.