Popular Woodworking 2009-10 № 178, страница 53

Flexner on Finishing

BY BOB FLEXNER

Five Common Finishing Problems

Bleeding, blushing, blotching, orange peel and fish eye.

I he basics of wood finishing are really quite simple: You use one of three tools - a rag, brush or spray gun - to transfer a liquid stain or finish from a can to the wood. Finishing becomes more complex when problems occur.

Here are five common problems, together with how to avoid them - and how to deal with them when they happen.

Bleeding

Bleeding refers to an oil finish oozing out of pores after being applied and wiped off. It is more likely to occur on large-pored woods such as oak or mahogany than on tight-grained woods. And it is more common with thinned commercial blends of oil and varnish (Watco Danish Oil, for example) than with pure oils such as boiled linseed oil or tung oil.

Bleeding is also more likely on hot days, especially ifyou move the wood into warmer temperatures or sunlight before the finish has totally cured.

If you allow the bleeding to dry and harden, it will form glossy scabs that can't be removed without also removing (by abrading or stripping) the finish around each. Sometimes, however, you can disguise the scabs adequately by rubbing the surface with #0000 steel wool then applying another coat to even the sheen.

To prevent the scabs from forming keep a close eye on your project and wipe over the surface with a dry cloth every half hour or so until the bleeding stops.

Once the wood is sealed, meaning the first coat has cured, there shouldn't be any more bleeding. So bleeding is usually limited to the first coat.

Blushing

Blushing is a milky whiteness that occurs in fast-drying shellac and lacquer finishes in humid weather. It's caused by moisture in the air condensing onto the finish as the solvents evaporate and cool the surface. The moisture then evaporates, leaving air voids that refract light rather than let it pass through.

Blushing doesn't occur in varnish because it dries so slowly, or in water-based finish.

To avoid blushing you have to slow the drying of the finish. Do this by adding lacquer retarder to the lacquer or shellac. (Brushing lacquer has already been retarded enough so that blushing is very rare.)

Adding retarder slows the drying of the finish, so don't add more than needed. You will have to experiment to find this amount because retarders use different solvent formulas and humidity can vary.

CONTINUED ON PAGE 68

Bleeding. The shiny spots on this oak panel show where an oil/varnish blend has oozed out of the pores and dried on the surface. To keep this from happening, check your project every half hour or so after you apply the finish and wipe off any bleeding before it dries.

Blushing. The milky white area in the center of this panel is called "blushing" and occurs often during the application of shellac or lacquer on humid days. To keep it from happening, slow the drying of the finish by adding a little lacquer retarder to it.

Blotching. The darker spots on this oak panel are areas where the stain dried before it was wiped off. To keep this from happening with fast-drying water-based and lacquer stains, work faster or on smaller areas at a time, or get a second person to wipe off quickly after you apply the stain.

66 ■ Popular Woodworking October 2009

PHOTOS BY THEAUTHOR