Hardwood Floor Finishing - Most Common Mistakes
1. Not sanding after every coat
2. Not stirring the finish properly.
There are several different types of protective finishes. The most common are penetrating sealers or surface finishes, including polyurethane, varnish, and shellac. Penetrating sealers (mentioned under staining) penetrate the pores of the wood, so the finish wears as the wood wears and can be retouched with wax in heavy-traffic areas. They come tinted or clear. These penetrating sealers are often used as an undercoat with surface finishes; but be sure the two finishes are compatible before you begin.
Surface finishes provide a tough, clear coating over stained or sealed wood. Polyurethane has pretty much replaced varnish, shellac, and lacquer. Often a heavy-traffic wax is applied over the polyurethane.
Finishes come in a clear gloss finish (high gloss) and a satin finish (low gloss). The high gloss shows dust easier.
Tip: Test a small area of the floor before applying stain. Often the sample stain at the store will give a very different effect when used on your large and particular floor.
Be sure to buy the slower-drying air-drying polyurethanes. Novices will have trouble with the faster-drying moisture-cured types that professionals use. Be sure whatever type you use is compatible with any undercoat you may have.
When working with polyurethane, be sure that it is well stirred before you apply it The hardeners settle in the bottom and, if it is not well stirred, the floor will not dry evenly. Use a paint stick or install a bent coat hanger in a drill that has a variable speed. Stir at a low speed so as not to create bubbles in the polyurethane. Ventilate the room well and wear a mask made for use with toxic fumes. In ventilating, however, be sure you're not creating a dust problem that will cause dust particles to settle on the wet floor.
It is best to apply the finish with a lamb's wool applicator and a paint tray. A brush is used at the walls and in hard-to-reach areas. Apply the polyurethane evenly, moving the applicator in the direction of the grain.
Usually three or four coats are applied. You will need to sand with a fine grade sandpaper between each coat after they have dried. The drying time will differ according to the humidity and temperature. Be sure you don't begin sanding until the previous coat is entirely dry, with no tacky feel. Sand with a professional buffer equipped with a #2 steel wool disc. This is much easier than using a hand sander. Hard-to-reach areas should be sanded by hand. Vacuum after each sanding, and then go over the floor with a damp mop to remove all the dust. The second coat can be applied across the grain. The final coat does not need buffing.
After your final coat wait until it has dried, and move in. For a few days the protective finish may "outgas" fumes, which may be rather unpleasant; but aside from that, the fumes should be gone after a week or so.