When learning how to apply polyurethane to wood flooring, what you use as well as your technique will determine whether there will be bubbles in your polyurethane layers. When learning how to put polyurethane on wood floors, you need to start with the right applicator, such as a finish brush and not an ordinary paint brush. A finish brush will apply a thinner coat than a paint brush, which would deliver globs of product. The thinner the layer, the less chance you will have of creating bubbles. Before you brush on the polyurethane, soak the brush in mineral spirits. You can also use water. This will displace the air caught between the bristles, so it doesn’t create bubbles once you start applying the polish. Gently brush it across a piece of newspaper to dry it out before use. Thinning the polyurethane can also help. Use mineral spirits for oil-based polyurethane and water for water-based products. Use a ratio of three parts polyurethane to one part of your thinning material. Gently stir in the solvent; never shake the products to mix, because shaking will create bubbles even before you start. When you start applying the polyurethane, practice applying thin layers in gentle strokes.