Sanding wood often requires coarse-grain paper to remove defects and generally smooth the wood. Just before finishing, however, most woodworkers and painters switch to a finer-grit paper for a final pass, and they use an even finer one to scuff between finish coats. Finish sanding makes a big difference to your painting or finishing job, but as far as wood is concerned, the benefits of using super-fine paper are limited.
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Sandpaper Grading Systems
Three sandpaper grading systems exist in the United States, but woodworkers normally need only be concerned with the one set by the Coated Abrasives Manufacturers Institute. The grit number on sandpaper sold in stores adheres to this system. The other important grading system, which is regulated by the Federation of European Producers Association is virtually identical for grits below 240, but it diverges for finer grits. Sandpaper graded according to the FEPA system has a P in front of the grit number, and a large FEPA number corresponds to a significantly smaller CAMI number at grits over 240.
Testing Fine Grits on Wood
Woodworker Ari Tuckman conducted a test on three wood species, using a variety of grits to prepare three species of wood -- oak, cherry and tiger maple -- for finishing with three clear finishes -- penetrating oil, shellac and oil-based polyurethane. He noticed virtually no difference on oak finished with shellac or polyurethane, no matter which grit he used for pre-sanding. He did notice a difference when finishing with oil, and he also noticed that cherry and tiger maple are more blotch-prone. He concluded that finish sanding with P150 sandpaper was sufficient for a film finish on most woods, and when finishing with oil, P220, or P400 for blotch-prone wood. P400 corresponds to 320-grit in the CAMI system. To summarize:
- Hand-sand wood with 150-grit sandpaper before finishing with polyurethane, shellac or lacquer.
- Continue sanding by hand with 220- and then 320-grit paper before finishing most wood species with oil.
Achieving a smooth surface on the wood isn't as critical when using paint, because the relatively thick surface hides minor scratches under an opaque coating. Adhering to the same standard as that for polyurethane guarantees good results, but in most cases, preparing the wood with 120-grit paper is sufficient.
Scuffing Between Coats
Hand-sanding between coats is the best way to ensure a smooth final finish, and there really isn't any upper limit to the grit you should use -- it all depends on your project. When painting wood with latex or oil-based paint, scuffing with sandpaper finer than 220-grit is likely to gum up the paper, and will have little effect on the final appearance. In most cases, a light scuff with 150-grit paper removes all surface defects without producing noticeable scratching. When painting wood with lacquer, shellac or polyurethane, a between-coat scuff with 320-grit paper knocks down roughness in the finish and levels drips that have had a chance to dry. You usually don't have to go higher than that, unless you're trying to achieve an ultra-smooth finish on medium density fiberboard or metal. In that case, wet-sanding is usually recommended, and you would go through the grits from 400-grit to as high as 1,000- or 1,200-grit until you're satisfied with the finish.