Wood Finishing

Tips & Techniques

How Fine to Sand

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 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.

 

Fine Sanding

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.

 

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.

 

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 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.

 

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 of practice to learn to control, there are three methods of sanding wood: with just 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.

 

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.I made my own sanding block. Its measurements are 2 3/4″ x 3 7/8″ x 1 1/4″ 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 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.

 

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.

Tack rags load up too quickly with the large amount of dust created at the wood level. These sticky rags should be reserved for removing the small amounts of dust after sanding between coats of finish.

 

How Much to Sand

The biggest sanding challenge is to know when you have removed all the flaws in the wood and then when you have removed all the scratches from each previous grit so you can move on to the next. Being sure that these flaws and scratches are removed is the reason most of us sand more than we need to.

A lot of knowing when you have sanded enough is learned by experience. But there are two methods you can use as an aid. First, after removing the dust, look at the wood in a low-angle reflected light – for example, from a window or a light fixture on a stand. Second, wet the wood then look at it from different angles into a reflected light.

 

Source: http://www.popularwoodworking.com/techniques/rules_for_sanding_wood

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