Tips for applying polyurethane varnish include choosing the proper brush, dampening the brush with solvent, pouring only what is needed into a separate container, applying the polyurethane in the direction of the wood grain and tapping the brush to release excess product. These tips are appropriate for both oil and water-based polyurethane finishes.
Depending on what base of polyurethane is being used, choose either natural bristle or synthetic fiber brushes. Use natural bristle brushes for oil-based polyurethane and synthetic fiber for water-based finishes. Synthetic brushes are made of polyester, nylon or a poly-nylon blend. Avoid using foam brushes or any type of roller, as they create bubbles in the finish.
When using an oil-based polyurethane, begin by dipping the brush into mineral spirits. This increases the smoothness and longevity of the brush and also makes it easier to clean. For water-based polyurethanes, simply dip the brush into water before applying the finish.
To keep dust, dirt and other debris from contaminating the can of polyurethane, pour small amounts of the finish into a jar or paint tray as needed. Avoid scraping the brush bristles on the edges of the container, which leaves the brush with too little product for a proper coat of finish. Tap the brush against the inside of the container, or gently hand-roll the brush handle to release excess product.
Always apply polyurethane of any type in the direction of the wood grain. This method works the product into the pores of the wood and creates an even appearance.