When I brush polyurethane finish I always end up with brush marks in the finish. What can I do to solve this problem?
Most oil based polyurethane finishes are too thick to brush straight out of the can. Thicker finishes take longer to "flow together" and even out. The problem you are having is that the brush marks aren't flowing together before the finish dries.
The solution to this problem is to thin the finish 10%-15% with mineral spirits or paint thinner. The thinned finish will take longer to cure and allow more time for the brush marks to disappear and bubbles to escape.