I have used Glassline paints with marginal results. I have a hard time getting good coverage of the glass. I recently purchased some Kaiser Enamels (http://kaiserglass.com/page32.html) and I have been really pleased with the results. They seem to have more “body” than the Glasslines, so coverage is much better (and thus, less chance of washout).
Jester’s Baubles Fused Glass Designs