Things You'll Need
Foam paint brush
Bristle paint brush
Paint stir stick
When brushed on wood, metal or other surfaces, or on top of a layer of paint, polyurethane forms a seal, providing a high degree of surface protection. Polyurethane typically leaves brush marks from a paintbrush whenever it is first applied, but if it is the right consistency and if the proper procedures are applied, the ridges resolve themselves and the polyurethane settles to a smooth finish. When the brush marks don't resolve themselves, try a few corrective techniques.
Use a foam brush or a lambswool applicator instead of a brush. These are not as likely to leave marks as a synthetic-bristle or natural-bristle brush.
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Use a natural-bristle brush rather than a synthetic one if you are using a bristle brush. Natural-bristle brushes are suitable for oil-based paints, and leave fewer marks than a synthetic brush.
Thin your polyurethane with paint thinner, mixing it with a paint stir stick. When polyurethane is too thick, the brush marks hold their shape and don't settle out. Thinning the product provides a smoother finished surface. Experiment with different amounts of paint thinner to see what works best for you. Some homeowners and construction workers use an equal amount of thinner and polyurethane in their mixture, but usually less thinner is needed.
Refrain from dragging your applicator back and forth quickly when you are applying the polyurethane. Not only does this create more bubbles in the applied product, but it also causes the product to dry faster, which gives brush marks less time to level out naturally.
Sand the dried surface. Let the first coat dry overnight, and sand it with a 280-grit or finer sandpaper. Apply successive coats of polyurethane, sanding between each application as it dries. After the final coat, sand with 600-grit sandpaper for a sleek, smooth surface.
If the room is too dry and too hot, the polyurethane will dry too fast, making it difficult for brush marks to resolve themselves.
Apply polyurethane in a well-ventilated area.