I had a poor set of wheels with a bad seam that was hard to get rid of. I actually used little bit of lacquer thinner on a q-tip and rubbed it lightly over the seam. Be very careful if you try this because the lacquer thinner basically 'melts' the rubber. If you're careful, you can melt the seam away and a light 'scuff' with sandpaper seems to finish everything off.
Murray