Popular Woodworking 2007-10 № 164, страница 66

Flexner on Finishing

How Fine to Sand

It's rarely beneficial to sand finer than #180 grit.

Film-building finishes, such as varnish, shellac, lacquer and water-based finish, create their own surfaces after a couple of coats. The appearance and feel of the finish is all its own and has nothing any longer to do with how fine you sand the wood.

Oil and oil/varnish-blend finishes have no measurable build, so any roughness in the wood caused by coarse sanding telegraphs through. But these finishes can be made ultimately smooth simply by sanding between cured coats or sanding each additional coat while it is still wet on the surface using #400-or #600-grit sandpaper. It's a lot easier doing this than sanding the wood through all the grits to #400 or #600. (See "What Is Oil?" in issue #154, April 2006, for a more thorough explanation of both processes.)

Only if you are staining or using a vibrator ("pad") or random-orbit sander does sanding above #180 grit make a difference.

The finer you sand, the less stain color will be retained on the wood when you wipe off the excess. If this is what you want, then sand to a finer grit. If it isn't, there's no point going past #180 grit. The sanding scratches won't show as long as they are in the direction of the grain.

Sometimes with vibrator and random-orbit sanders, sanding up to #220 grit makes the squiggly marks left by these sanders small enough so they aren't seen under a clear finish. Sanding by hand in the direction of the grain to remove these squigglies then becomes unnecessary.

In all cases when sanding by hand, it's best to sand in the direction of the wood grain when possible. Of course, doing this is seldom possible on turnings and decorative veneer patterns such as sunbursts and marquetry.

Cross-grain sanding scratches aren't very visible under a clear finish, but they show up very clearly under a stain. If you can't avoid cross-grain sanding, you will have to find a compromise between creating scratches fine enough so they don't show and coarse enough so the stain still darkens the wood adequately. You should practice first on scrap wood to determine where this point is for you.

Three Sanding Methods

Other than using a stationary sanding machine or a belt sander, which will take a good deal

Hand sanding. The most efficient use of sandpaper when backing it with just your hand is to tear the sheet into thirds crossways and then fold one of the thirds into thirds lengthways. Flip the thirds to use 100 percent of the paper.

of practice to learn to control, there are three methods of sanding wood: withjust your hand backing the sandpaper, with a flat block backing the sandpaper and with a vibrator or random-orbit sander.

Using your hand to back the sandpaper can lead to hollowing out the softer early-wood grain on most woods. So you shouldn't use your hand to back the sandpaper on flat surfaces such as tops and drawer fronts because the hollowing will stand out in reflected light after a finish is applied.

The most efficient use of sandpaper for hand-backed sanding is to tear the 9" x 11" sheet of sandpaper into thirds crossways, then fold each of these pieces into thirds lengthways. Sand with the folded sandpaper until it dulls, flip the folded sandpaper over to use the second third, then refold to use the third third. This method reduces waste to zero and also reduces the tendency of the folds to slip as you're sanding.

If you are sanding critical flat surfaces by hand, you should always use a flat block to back the sandpaper. Ifthe block is hard (wood, for example), it's best to have some sort of softer material such as cork glued to the bottom to improve the performance of the sandpaper. (I find the rubber sanding blocks, available at home centers, too hard, wasteful of sandpaper and inefficient because of the time involved in changing sandpapers.)

I made my own sanding block. Its measurements are 2³A" x 3%" x 1V4" thick, with the top edges chamfered for a more comfortable grip. Any wood will work. I used sugar pine because it is very light in weight.

To get the most efficient use of the sandpaper, fold one of the thirds-of-a-sheet (described above) in half along the long side and hold it

Block sanding. The most efficient use of sandpaper when backing it with a flat sanding block is to tear the sheet into thirds crossways and then fold one of the thirds in half. Hold onto the block with your thumb and fingers as shown here. Flip the folded sandpaper for a fresh surface, then open up the sandpaper and wrap it all around the sanding block for a third fresh surface.

in place on the block with your fingers and thumb. When you have used up one side, turn the folded sandpaper and use the other. Then open the sandpaper and wrap it around the block to use the middle.

Most woodworkers use random-orbit sanders because they are very efficient, easy to use, and they leave a less-visible scratch pattern than vibrator sanders due to the randomness of their movement. For both of these sanders, however, there are two critical rules to follow.

First, don't press down on the sander when sanding. Let the sander's weight do the work. Pressing leaves deeper and more obvious squigglies that then have to be sanded out. Simply move the sander slowly over the surface of the wood in some pattern that covers all areas approximately equally.

Second, it's always the best policy to sand out the squigglies by hand after you have progressed to your final sanding grit (for example, #180 or #220), especially ifyou are applying a stain. Use a flat block to back the sandpaper if you are sanding a flat surface. It's most efficient to use the same grit sandpaper you used for your last machine sanding, but you can use one grit finer if you sand a little longer.

Removing Sanding Dust

No matter which of the three sanding methods you use, always remove the sanding dust before advancing to the next-finer grit sandpaper. The best tool to use is a vacuum because it is the cleanest. A brush kicks the dust up in the air to dirty your shop and possibly land back on your work during finishing.

80 ■ Popular Woodworking October 2007