How to Use Semigloss Polyurethane to Refinish a Pine Floor

Polyurethane Characteristics

• Polyurethane is a plastic that becomes hard through a cross-linking process. It cures as the solvent carrying the plastic evaporates and the plastic is exposed to moisture in the air. The resulting coating is highly reflective and can look uncomfortably artificial in strong light. To counteract this, manufacturers add polyethylene wax and other solids to mat the finish. The resulting finish behaves exactly the same as a gloss finish, but because it isn't as reflective, small defects in the finish or the wood underneath aren't as noticeable. It is therefore a more forgiving finish than a gloss one.

Pine Floor Preparation

• Before you apply a finish to pine, you need to sand the wood, even if the floor is new and boards have never before had a finish. Most professionals use a flooring drum sander and edger, but you can get very good results on pine with a flooring orbital sander, although it takes longer. Because pine is a softwood, you shouldn't ever have to sand with paper coarser than 50-grit, and the ideal preparation consists of subsequent passes with 80- and 100-grit paper. Thoroughly vacuuming the floor and wiping sanding dust with a damp rag gets it ready for finishing.

Staining and Sealing

• Sanding opens the wood grain, so a pine floor is very absorbent after you sand it. If you plan to stain, you should spread wood conditioner with a paintbrush first. It partially seals the grain and prevents blotching, which is always a concern with pine. After the stain dries, spreading a sealing coat is a recommended procedure. You can use a commercial sealer, which is often shellac-based, or spread a coat of dilute finish, which performs essentially the same function. Neither penetrates the wood, and both raise the wood grain, so you need to sand before you apply polyurethane finish. A floor buffer fitted with a 120-grit sanding screen works well for this job.

Spreading Finish

• Both oil- and water-based polyurethane finishes can form bubbles that harden into the finish. You can avoid most of these by spreading the finish with a flat or tubular applicator that you drag along the floor in the grain direction, generating as little turbulence as possible. Screening the floor after each coat knocks down any bubbles that do form, as well as flattening any other defects. Most floors look best after a minimum of three coats of finish; only the last coat needs to have matting additive for the floor to have a semigloss sheen.