Sanding is not the most pleasant woodworking task but it goes a lot smoother (pun intended) if you have a system in place for choosing sandpaper grits. That is: what grit to start with, what grit to end with, and what grits to use in between. This process is sometimes referred to as a sanding schedule or sanding sequence or more simply, going through the grits.
Keep in mind that the purpose of sanding is to remove mill marks on the wood created by woodworking machinery – planers, saws, jointers, etc., as well as any scratches, dents, burns, and other defects. This is most efficiently done by first sanding the wood with a coarse enough grit to remove the blemishes. Then, it’s a matter of sanding the wood with progressively finer and finer grits with each grit removing the scratches from the previous grit and replacing them with denser but shallower ones. Eventually the wood will reach the desired smoothness. That’s the gist of going through the grits.
Quick aside: If one is proficient with hand planes and scrapers, it may not be necessary to sand at all. These simple tools can work wonders. Make a few quick passes to remove the mill marks and other defects and move on to the finishing. End of story. But hand tools aren’t for everyone and some tasks are easier done with sandpaper. So it’s onto the grits…
Begin sanding with the finest grit that’s capable of removing the machine marks and other imperfections on the wood. In most situations, that will be 80 to 120 grit. Use your best judgment and experience here. If the milling marks are not very obvious, you can usually start at 100 or maybe even 120. It’s counterproductive to start at 80 grit in many situations. That will just put down deep scratches that you then have to remove. But if you have deep gouges in the wood or it has pronounced “washboard” ripples from the planer, you may well have to start sanding at 80 grit.
Vacuum off the dust from the first sanding pass and move onto the next grit. This will be the next finer level of coarseness — 150 or 180 grit (if you started at 80 to 120 grit). Don’t be tempted to “skip a grit” by going right to 220 or higher. That will actually take longer because the finer grit will be much less effective at removing the initial sanding marks.
Perform a final sanding pass using a very fine grit – 220 to 240 (I prefer 220 grit). In most cases, the wood will be ready for finishing after this third pass. If you sand beyond this level, the wood will look more polished but after applying the finish, the polish goes away. A 400 grit surface won’t look appreciably different than a 180 grit surface. Test this yourself the next time you’re finishing a project. Note that in addition to wasting your time, excessive sanding can also impair the absorption of certain stains.
How do you know when to stop sanding? This is something you gain with experience but there is a simple way to determine if more sanding is required. First, vacuum or wipe off the sanding dust. Then, look at the wood surface with a low angle light. If you see any residual scratches, keep sanding. It can also help to lightly dampen the surface with mineral spirits or water.
Summary: A good general rule for grit progression is to use 100 or 120 grit for initial sanding, 150 or 180 grit for the next pass, and 220 grit for the final pass.
Sandpaper Abrasive Selection
In terms of sandpaper abrasives, a common approach is to start sanding with aluminum oxide sandpaper using a power sander and then switch to garnet paper for final sanding by hand. Aluminum oxide lasts longer than garnet because it is harder but it doesn’t break down as well so the sharp edges become dull. This can result in burnishing of the wood surface which affects penetration of the finish. Garnet continuously breaks down to expose fresh sharp edges (it is friable) but it is softer than aluminum oxide and thus better suited for hand-sanding. Silicon carbide sandpaper works well for sanding the finish because it is usually stearated or lubricated, preventing it from clogging up so quickly.
What the Experts Do
So, what do the wood finishing experts do when it comes to sanding wood? Well, here’s what Bob Flexner, Michael Dresdner, Jim Tolpin, and Jeff Jewitt have to say:
Flexner generally starts at 80 or 100 grit and seldom goes past 220. He prefers to sand to 180 when applying a film finish (shellac, lacquer, varnish, conversion, or water-based) and 220 when applying a thin oil finish. Sanding to 200 grit and above will polish the surface and hinder pigment-stain penetration. States that most furniture manufacturers seldom sand above 150 grit.
Dresdner offers the following rule of thumb for grit progression: jump under 100 points with coarse papers, 100 or more with medium, and 200 or more with very fine papers. Example grit progressions: 80/120/220/400 or 100/180/280/400. When in doubt, better to have too many steps than not enough.
As far as how fine to go, Dresdner says that 180 grit is usually sufficient for most woods, and in some cases, 120 grit. But a finer grit may be required for very hard woods like ebony or boxwood. Light colored woods like maple may require 220 grit for certain stains. If you can see the scratches, go finer.
Jewitt cautions against starting with too coarse a grit. A piece that has been scraped or planed to near perfection won’t require 80 grit for the first pass. Instead, it might be possible to start with 180, followed by 220. States that sanding beyond 180 or 220 isn’t necessary, especially for a film finish and he never goes above 240 grit for an oil finish. No hard and fast rules for grit progression other than to sand until the scratches left by the previous grit are gone.
Jewitt describes a test where he sanded a board using six different grits and applied both a lacquer film finish and a linseed oil finish. After several weeks, the differences in appearance between the different grit levels were not all that apparent, especially for the film finish. He says that a penetrating oil finish may telegraph a slight texture difference from sanding so going to a slightly higher grit can be justified in such situations. (but he still doesn’t go beyond 240 grit).
Unless the milling marks are really pronounced, Jim Tolpin suggests starting off with 100 grit aluminum oxide “production” paper or possibly even stearated silicon carbide paper. For clear finishes on solid wood, his recommended grit sequence is 120, 150 or 180, and finally 220. He reiterates that if you go beyond 220, you’ll burnish the surface, which interferes with the adhesion or absorption of many finishes.