I used a brush to apply
polyurethane on some trim work. I tried cleaning it thoroughly with
paint thinner after but still got poor results--the brush bristles would stick together and the brush was unusable. I also heard
mineral spirits and
acetone may work. Should I try something else or are brushes pretty much done after a single use?
I used a brush to apply
I have had trouble cleaning them with everything and had the same problem, until I found a jar large enough to put the brush in filled to the bristle level with thinner keeping it tightly sealed. (I do clean them prior to putting in the jar). When I need them take out shake dry and they work like new brushes for months without being used or when I have a project that gets a new coat every day for a week. (I find the higher quality brushes provide the best coverage so I hate to toss them with just 1 use)
I make picture frames and I like polyurethane because it is a very hard, durable finish. I always use foam brushes. To clean I merely soak in paint thinner a short time shake it out and dry with a paper towel. You can do this over and over if you want. But at around twenty five cent for a brush you could just toss them. And they leave a nice even coat as well.
since there are water based polyurethanes that you can clean up with water, you can use those. however, start with a good quality brush. if you want to use an alkyd, clean (thoroughly - not just a dunk) with paint thinner (takes the paint into solution and flushes it away), then with acetone (takes the thinner into solution and flushes it away), then with water (takes the acetone into solution and flushes it away as well as acting as a solvent for the soap). then rub the bristles in hand bar soap (i like ivory hand soap for this). the brush will stay nice and soft and supple. when you want to use it again, just quickly dunk it in whatever solvent is the solvent in whatever paint you want to use. you can keep a quality brush going for years with this approach.
Maybe not politically correct but I found a quick dunk & swoosh in gasoline works better then any paint thinner or solvent. After dunking, I squeeze out what I can by hand & then apply liquid dish soap (like dawn) & work into the bristles to remove the gas. Rinse in hot water & allow to dry. I find that placing a rubber band on it helps to keep shape.