Polyurethane is a synthetic finish. Unlike stains, that penetrate porous surfaces, finishes remain on the wood's surface where they provide a glossy layer. The glossy layer is transparent but can slightly darken the wood's appearance. Determine the amount of color change the wood may undergo after polyurethane application by brushing denatured alcohol over the wood's bare surface. The resulting damp appearance will be similar to the wood's shade after the polyurethane application.
Oak wood may appear white after the polyurethane application if the finish is applied too thickly, if bubbles are present in the finish or if debris has contaminated the surface of the wood. Avoid these problems by, first, prepping the wood's surface. Wipe the surface with a tack cloth to remove any dust. Vacuuming the surface is also effective. If your work area is full of dust or wood shavings, apply polyurethane in a different room with clean surfaces. Polyurethane should not be used straight out of the can. Instead, pour a small amount of polyurethane into a clean container, and protect the remaining mixture from dust by covering the original container. Replenish the secondary can as needed. Some brands require stirring prior to application. Follow the manufacturer's specific instructions. A clean surface and a uniform mixture help ensure the wood's coloring isn't compromised.
Avoid thick applications and cloudy finishes by using only the tip of a clean brush. Only use the upper one-third of the brush. This diminishes the risk of introducing air bubbles into the finish mixture. When air bubbles dry, they appear as whitish dents on the wood's surface. Dip the brush into the polyurethane and apply in quick, smooth strokes with the wood's grain. Immediately after the first coat is applied, apply the second coat across the grain. The final coat is light and once more with the grain. Allow the finish to dry. Applying the finish both with and across the wood's grain prevents polyurethane buildup, and results in a clear, glassy, protective coat.
Other factors, such as humidity, heat and cold, also affect polyurethane's appearance. A controlled 70-degree F, low-humidity environment is the optimum climate for polyurethane application.