I avoid sealing polymer whenever I can, preferring to sand and buff for a natural shine. But a lot of my techniques involve surface effects with paint and ink transfers, so sealing is necessary to protect the finish. A couple of years ago I discovered PYMII and what a difference it has made!
It sprays on, so no more brush strokes and globs. It dries quickly, so that you can apply multiple coats without having to wait all day. And best yet, it dries to an even, glossy shine. I've found that the trick is to put your piece back in the oven for 5 or 10 minutes to heat set the PYMII. The result is a rock hard, durable and waterproof finish.
Here are some pieces sealed with PYM II: