Popular Woodworking 2008-08 № 170, страница 62

Slant paneli Slanting a panel 5" or 10° is not the way to reduce brush marking, but this has actually been suggested recently. You would have thought furniture finishers working during the last several hundred years would have figured this out if it were true.

Brush marks. The way to apply a brush-mark-free finish using polyurethane is to first sand out the brush marks on the next-to-last coat using a flat backing block and a mineral oil, mineral-spirits or water lubricant. Then thin the last coat about 25 percent with mineral spirits so it levels

It's gravity evening out the di fference between the ridges and the troughs that eliminates brush marking.

Some brands of polyurethane level naturally better than others because of their formulation. But all polyurethanes can be made to level perfectly by adding mineral spirits. So the way to get a brush-mark-free finish is to sand the next-to-last coat level, then thin the last coal enough so it levels well.

The amount of thinner necessary varies with brands. My suggestion is to begin with about 25 percent mineral spirits and adjust from there.

■ MYTH #7: Scuff sand between coats to get a good bond. The purpose of this instruction is to create scratches in the surface so the next coat of finish can establish a "mechanical" bond. The finish "keys" or "locks" into the sanding scratches.

This myth is somewhat complicated. The first clue that sanding between coats isn't so critical is that you rarely create scratches everywhere anyway. There are almost always gaps in your sanding - for example, in the pores, in recesses and often just because you aren't being thorough enough.

And yet, the next coat usually bonds well anyway, especially if not a lot of time has gone by between coats. How often have you seen coats of polyurethane separating?

Here's the way to approach sanding between coats: Do it anyway. Polyurethane dries slowly, so there are always dust nibs that should be sanded out before the next coat is applied.

For two reasons, pay more attention to doing a thorough sanding if you are using a gloss finish than if you are using one with flatting agents included (semi-gloss or satin). First, even tiny flaws show in gloss finishes, while they are often disguised in semi-gloss and satin. Second, finishes don't "wet" and bond as well to gloss surfaces as they do to dull surfaces. (The dullness insemi-gloss and satin finishes is caused by a microscopically rough surface created by the flatting agent.)

Use a sandpaper grit that removes the flaws efficiently without creating deeper scratches than necessary. I almost always sand with #320 or #400 grit, regular or"P" grade. There have to be big flaws in the surface to require sanding with coarser grits.

I never back the sandpaper with a flat block when sanding between coats to remove dust nibs, though it would be all right to do this if the surface is truly flat. The biggest problem using a flat block is that you are more likely to clog the sandpaper and the clogs, or "corns," will put deeper scratches into the finish that might telegraph through the next coat.

You can also abrade with steel wool or a synthetic abrasive pad, but neither cuts and levels as well as sandpaper.

Conclusion

Brushing polyurethane is not complicated, but somehow authors and manufacturers have succeeded in making it seem so. They have done this by introducing myths into

Scuffing. The main reason to sand polyurethane between coats is to level dust nibs and other flaws, not to create a better bond. This is demonstrated here by the gloss remaining in the pores after sanding the surface. The next coat of finish still bonds in the pores even though the finish there is rarely scuffed.

their instructions. Some of the myths make no sense but do no harm. Others purport to solve a problem, and when following them doesn't help, frustration sets in.

It's unfortunate, but once a myth gels into print, nothing, noi even an article such as this, seems to have any impact slowing its spread.

Probably the opposite, in fact. Conspiracy theorists know that merely repeating an "untruth" in order to debunk it tends more to solidify it in the minds of believers. PW

Bob is the author of "Understanding Wood f inishing and a contributing editor to this magazine.