Properly Sanding Your Project

by Bill Tindall

To achieve a good looking finish, proper preparation of the wood surface is essential. Hand planing and scraping were traditionally used to prepare the wood surface for finishing, but today most woodworkers use sanding. This article will describe some tips to get a perfectly sanded surface.

Prior to sanding, the wood will have been machined in some way. The surface may have been planed, jointed or a profile routed. The resulting surface will have some marks from the cutter, often a series of ripples, and it will be burnished and glazed, especially if the cutter was at all dull. Ideally, the only objective in sanding will be to smooth the tiny ripples and remove the glaze. However, machining defects may leave the surface uneven and there can be areas where the cutter picked out wood or tore the grain. Eliminating these problems will involve sanding away enough wood to get below these defects. The entire surface must be sanded to keep it flat. If the defect is deep enough it will be impractical to sand it out and it must be filled. Fixing these deep defects requires a great deal more experience than simple surface preparation. Hence, preventing these defects should be a prime objective when machining. Hand planing, scraping, or machine sanding with a belt sander is usually necessary to remove deep machining defects. These operations will not be described. The following description mainly covers sanding to smooth minor machining defects and glaze.

When sanding I use the "80/20 rule", 80% of the time is spent on the starting grit size and 20% on all the remaining sizes. For end grain the rule is 90/10. Start with the finest grit that will fairly quickly get the surface flat. For a well machined surface 120 grit will do. On something exceptionally hard like oak, or a more poorly machined surface I may start with 100 grit. I try to avoid using 80 grit, although it can sometimes be necessary. The scratches from 80 grit can be hard to remove with subsequent grits.

All the machining defects and glaze must be removed by sanding with the initial grit, especially in the case of end grain sanding. If the surface is still unacceptable after a few minutes of sanding with the initial grit chosen avoid the temptation to continue with this grit size. Switch to the next coarser grit. Subsequent finer grits will never fix a problem left by the initial grit. These defects will show up for all to see when the stain and finish are applied. Beginning woodworkers often fail to appreciate this fact. Of course, always sand with the grain. Cross grain scratches are very hard to remove and will become prominent on a stained piece.

Woodworkers often discover defective sanding only when they stain, and then it is too late!. The improperly sanded areas take excessive stain and appear blotchy. So, how do you know when you have sanded enough? It really is very easy. Look for dull places and shiny places. A properly sanded surface will be uniform in luster-uniformly dull with the coarser grits and uniformly more shiny with the finer grits. In the beginning the places that aren't sanded will be shiny from the machining glaze. When these places are sanded away, look for dull areas. If there is torn grain from machining, these areas will trap sanding dust, which will make them dull looking compared to the properly sanded surrounding areas. These same areas will soak up stain and be plainly visible as blotches on the finished surface. This dull vs less dull appearance will be especially plain to see on end grain. Insufficiently sanded end grain will appear very dull compared to properly sanded end grain. Even after 100 grit sanding end grain will appear shiny and uniform and it will take stain uniformly.

Only when the entire surface is free of dull places that result from torn grain, or shiny places from machining glaze, can the next finer grit be used. Most of the work has been done by the initial grit. The subsequent grits have the easy job of just removing scratches from the previous grit. To make the subsequent sanding go quickly, never skip more than one grit size, and in the coarser grades, skip no grit sizes. So for example, suppose in the worse case one began with 80 grit to remove some severely torn grain. (More typically I will begin with 100 grit, or even 120 on a well-machined surface) Eighty grit would be followed by 100 and then 120 grit. After 120 it is acceptable to skip 150 and go to 180. For many projects it may be sufficient to stop at 180. I never sand oak beyond 180. For very plain featureless woods like maple I will sand to 220.

These recommendations imply that one needs to stock 100, 120, 180 and perhaps 220 and 80 grit sand paper. Abrasive manufacturers recommend aluminum oxide for wood sanding. The black silicon carbide is unacceptable because the black grits can stick in the pores of light woods and discolor them. There are uses for garnet, silicon carbide and aluminum oxide-zirconia paper, but it is beyond the scope of this article to describe these applications. When in doubt, just use aluminum oxide.