Popular Woodworking 2008-08 № 170, страница 61

—■ Flexner on Finishing ■—

Sticky stage. Once the bubbles have popped out, you want the polyurethane to dry as fast as possible to get beyond the sticky stage (demonstrated here by a fingerprint in the finish) so dust nibs are kept to a minimum. Adding faster-evaporating naphtha to the finish doesn't speed the drying of this stage beyond what is accomplished simply by the thinning. You could just as well use mineral spirits.

the finish "open" longer and give the bubbles more time to pop out.

Some brands of poly urethane, such as Min-wax, bubble noticeably more than others, but this is rarely a problem because the bubbles tend to pop out quickly.

There are two ways to deal with bubbles that won't pop out. The first is to wipe off the bubbled coat of finish using a rag dampened with mineral spirits, naphtha or turpentine. You can do this for 15 to 30 minutes after application without a problem. You won't damage the coat underneath, and you don't have to get all the polyurethane removed. Simply wipe until what is left is smooth and bubble-free.

The second method is to let the bubbled finish dry and sand it level before applying another coat. Thin this coat enough with mineral spirits so the bubbles have time to pop out.

The disservice caused by attributing bubbles to shaking rather than to brushing is that users become frustrated and lose confidence when they follow directions and still don't avoid the problem.

■ MYTH #4: Thinning with naphtha makes polyurethane dry faster. Naphtha evaporates much faster than mineral spirits or turpentine. So the logic behind this myth, which is fairly new, is that the finish will dry faster if the thinner evaporates faster.

But, like all varnishes, polyurethane dries in two steps. The first is evaporation of

the thinner. The second (and much longer) step is the curing, which is the crosslinking brought about by the introduction of oxygen from the air.

Whenyou apply polyurethane, you notice that it stays wet on the surface for a short time as the thinner evaporates. Then the finish goes into a tacky or sticky stage for an hour or longer. This is the length of time it lakes for the oxygen-induced crosslinking to occur. Adding a faster-evaporating thinner doesn't speed this crosslinking.

In fact, adding naphtha probably has no noticeable effect on the drying of the finish beyond the impact of thinning described at the end of the second myth.

■ MYTH #5: Thinning with Penetrol reduces brush marks. Painters have added Penetrol, a widely available additive, to oil paint for decades to reduce drag and brush marking, especially when painting in hot or cold weather, or in sunlight. But until recently 1 had never seen Penetrol recommended for use in polyurethane, which is usually applied indoors in more ambient working conditions and brushes easily without drag.

Penetrol is a slow-drying oil product that lengthens the tacky stage of poly urethane and oil paint. This creates more time for dust to

settle and stick to the surface. Painters don't mind because dust isn't a big problem for them, but it is for furniture finishers.

Most woodworkers want their polyurethane to reach a dust-free stage faster, not slower, as evidenced by myth #4. What little benefit might be gained in reducing brush marking is more than cancelled out by increased dust nibs. (If you want to reduce dust nibs, thin the last coat of polyurethane by 25 to 50 percent so it dries faster.)

Additionally, because Penetrol is an oil, it can't help but weaken the durability of polyurethane - though not enough to cause a problem in most situations, so far as I can tell. (See the next myth for the way to eliminate brush marks.)

■ MYTH #6: Slant the panel to reduce brush marks. I read this myth for the first time only recently. The idea is to get brush marks to (low together by tilting flat panels such as tabletops 5° or 10° off the horizontal.

Not only is this a tricky procedure that will lead to sagging if you aren't careful to keep the finish thin on the surface (similar to the difficulty brushing vertical surfaces), but the procedure doesn't make any sense. It's not gravity causing one brush-mark ridge to sag into another that reduces brush marking.

Penetrol. Adding Penetrol (a widely available additive for oil paint) to polyurethane slows its drying so dust has more time to settle and stick to the finish. Any possible gain in reduced brush

than cancelled out by the added dust

78 ■ Popular Woodworking August 2008