True Grit: How to Choose Sandpaper Grit and Wood Finishes

The sandpaper grit you choose can make a big difference in the final look of your woodworking project. The project you are working on will determine the sandpaper grit you should use, as well as the best wood finish for the look you are trying to achieve.

| December 21, 2011

From the editors of Fine Woodworking magazine, Best Finishing Techniques (Taunton Press, 2011) is a firsthand report of various experiments and studies done by skilled woodworkers to answer your questions about the best way to finish up a project in your woodshop. The book covers sandpaper selection and how to sand wood, coloring and applying finishes to your products, and even finish recipes. The following excerpt is taken from Part Two, “Surface Preparation and Sanding: When to Stop Sanding.”  

Sanding is most woodworkers’ least-favorite activity. It’s dusty, boring, and time-consuming — the sooner done, the better. But what is the right stopping point? When does moving to a finer grit no longer yield appreciable improvements in the finished surface? To find out, I did a semi-scientific study. I took boards of cherry, white oak and tiger maple as test woods, not only because they are familiar furniture woods but also to see if tight or open grain and figure would influence the best final grit. I cut each into six pieces and sanded them to six different grits. I then divided each piece into thirds and applied a different finish to each section because what really matters is not how the bare boards look but their appearance with a finish. The results were both interesting and reassuring. 

Six Sandpaper Grits

I chose aluminum-oxide sandpaper graded to the FEPA scale (recognizable by the P prefix to the number) because it is the standard abrasive for sanding bare wood. I tested P120, P150, P220, P320, P400, and P600 grits because most of the sanding I had previously done was with either P220 grit or P320 grit. I wanted to see if coarser or finer grits would make a noticeable change in the finished appearance of the wood. Would the open-grained pattern on white oak conceal the scratches left by coarser grits? Would cherry, a blotch-prone wood, respond best to finer-grit sanding? Would the stripes of tiger maple be enhanced by a coarse final grit, or would they be left blotchy? 

I used a random-orbit sander for the majority of the sanding, progressing through the grits. This was followed by hand-sanding with a sanding block, with the grain, at the same final grit. I changed the disks when they began to wear out, but I used fresh paper on each board’s final grit for both the random-orbit and the hand-sanding. I chose these three woods to see if wood grain or figure would make any difference. I cut one long board of each species into six 171⁄2-in. by 8-in. sections and machine-planed them flat. Once the 18 sections were sanded to the appropriate final grit, I removed the dust using a clean paintbrush and a vacuum.  

Three Wood Finishes

After sanding, I used masking tape to separate each board into three sections. This allowed me to apply three different finishes to see if some are more sensitive to the final grit than others. I chose Danish oil for a minimal-build, in-the-wood finish; shellac rubbed out with steel wool and then waxed for a medium-luster, thin-film finish; and an oil-based polyurethane to give a more protective, high-gloss finish.  

I used natural Watco Danish oil, wiping on the first coat with a cotton cloth, then wiping off the surplus. When dry, I applied a second coat and wet-sanded using P400-grit wet/dry sandpaper and a sanding block. Then I wiped it dry to remove the surplus sawdust and oil. The next day, I applied a final coat in the same way as the first.

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