Popular Woodworking 2000-01 № 112, страница 13

Flexner on Finishing

Continued from page 24

but it's a disadvantage when applying a stain. Most water-based stains are difficult to apply evenly on large surfaces because they dry too fast to get the excess wiped off.

There are several tricks you can use to get an even color on a large surface. Spray the stain evenly and leave it. Spray the stain and wipe it off quickly. Wipe on the stain with a large soaked rag, and quickly wipe off the excess. Stain small sections at a time. Have one person apply the stain and another wipe off right behind. Add a manufacturer's solvent, usually propylene glycol, to the stain to slow the drying. (Unfortunately, very few manufacturers supply this solvent.)

If you do get an uneven coloring due to some of the stain drying before you get it wiped off, here's what you do: Quickly wipe over the entire area with a wet cloth to redissolve and respread the stain. However, you should be aware that this might lighten the color.

Bonding to Oil Stains

To avoid both the grain-raising and fast-drying problems of water-based stains, you can use an oil-based stain, but then you have to worry about the water-based finish not bonding well. To ensure a good bond, let the oil-based stain cure totally before applying the water-based finish. Curing time depends on the stain itself, the weather and the size of the wood's pores. For example, stain on oak will take longer to cure than on maple.

Some water-based finishes bond well over some oil-based stains before the stain has cured, but there's no way of knowing for sure without trying it. The variables are the resins and solvents used in the finish and how much oil is in the stain.

To test the bonding, apply the stain and finish to scrap wood. After drying for a couple of days, score the finish with a razor in a cross-hatch pattern. Make the cuts about apart and 1" long. Then press some masking or Scotch tape over the cuts and pull it up quickly. If the finish has bonded

well, the scored lines will remain clean and little or no finish will come off on the tape.

Other Problems

Water-based finishes have other problems as well, but they aren't serious enough to make you change to another finish except in special situations. These problems include the following: sensitivity to weather conditions (all finishes are sensitive in high heat, cold and high humidity); less durability than varnish or polyurethane (but more durability than shellac or lacquer); and greater difficulty repairing or stripping than shellac or lacquer.

The problems using water-based finishing products aren't insurmountable. But they're made more formidable by manufacturers who claim too much, mislabel their products and provide inadequate instructions for dealing with the special problems created by water. PW

Bob Flexner is a nationally known finishing expert in Norman, Oklahoma, and the author of "Understanding Wood Finishing."

22 Popular Woodworking January 2000