When finishing with spar urethane, thinning it is a very good idea to keep it from being too thick and showing off too many brush strokes. Some prefer to thin the urethane with paint thinner or mineral spirits before applying thin coats with a cloth. The disadvantage is that it requires twice as many coatings to apply properly, and bubbles in the urethane are also likely to cause problems during
. the application process. It is also possible to thin and spray the urethane onto the surface being finished. You can find more information here: http://thewoodwhisperer.com/a-better-way-to-apply-spar-urethane-question-of-the-week/
