Popular Woodworking 2007-10 № 164, страница 65

Flexner on Finishing

by bob flexner

Rules for Sanding Wood

Material and finish choice help dictate grit progression.

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I he objective of sanding wood is to remove mill marks, which are caused by woodworking machines, and to remove other flaws such as dents and gouges that may have been introduced in handling. The most efficient method of doing this is to begin sanding with a coarse enough grit of sandpaper to cut through and remove the problems quickly, then sand out the coarse-grit scratches with finer and finer grits until you reach the smoothness you want - usually up to #150, #180 or #220 grit.

This is a very important concept because it gets past all the contradictory instructions about which sandpaper grits to use. Conditions vary.

For example, a board that has been run through a planer with dull knives will require a coarser grit to be efficient than typical veneered plywood or MDF that has been pre-sanded in the factory. You can finish-sand both of these surfaces with #180 grit, for example, but you might begin with #80 grit on the solid wood and #120 grit on the plywood. It would be a total waste of time and effort to begin with #80 grit on the pre-sanded veneered wood (and you would risk sanding through). So you don't want to begin with too coarse a grit because it will cause you more work than necessary sanding out the scratches.

There's also no fixed rule for how to progress through the grits. Sanding is very personal. We all sand with different pressures, number of passes over any given spot and lengths of time.

Unquestionably, the most efficient progression is to sand through every grit - #80, #100, #120, #150, #180 - sanding just enough with each to remove the scratches of the previous grit. But most of us sand more than we need to, so it's often more efficient to skip grits.

You'll have to learn by experience what works best for you.

Washboarding. The primary reason you need to sand wood is to remove the washboarding and other mill marks caused by machine tools. On this board, the washboarding, which was caused by a planer and has been highlighted with stain, is particularly severe. I think it would have been most efficient to begin sanding with #80 grit.

Cross-grain. Sanding cross-grain tears the wood fibers so the sanding scratches show up much more, especially under a stain. The best policy is to always sand in the direction of the grain when possible. The scratching that does occur is then more likely to be disguised by the grain of the wood.

Squigglies. Random-orbit sanders are more efficient than vibrator sanders, but they still leave cross-grain marks in the wood. I refer to these as "squigglies." The best policy is to sand them out by hand in the direction of the grain after sanding to the finest grit, usually #180 or #220, with the sander. Doing this is especially important if you are staining.

Fine sanding. Sanding finer than #180 or #220 is wasted effort in most cases, as explained in the text. In fact, the finer the grit the wood is sanded to, the less color a stain leaves when the excess is wiped off. In this case, the top half was sanded to #180 grit and the bottom half to #600 grit. Then a stain was applied and the excess wiped off.

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