How old is your paint? That could be a problem, 2nd thing is are you using an "enamel" brush? Do you "put back" paint you don't use in bottle? Other than that can't think of anything. I notice paints do go bad so they have a "shelf life". Hope this helps.

Gosh, I was hoping more people here would hop in and give you some advice. I know there sure used to be lots of people painting on wine bottles, glass blocks, and glasses here in the past.

I've only tried painting on glass once or twice. I really didn't like the coverage the paint gives. I was using the outdoor/indoor gloss paint so thought that was why. (I didn't want to have to go buy new Permenamel or some other brand for just the two glasses I wanted to paint.)

I have read that you should apply the glass medium first then when it dries, apply your paint. I find that the first coat is very sheer, but after it dries, you can add one or two more layers for better coverage.

And Sunnyca is right, there are extra soft brushes just for glass painting. I think they are supposed to flow the paint on smoother and not leave lines.

Good luck with your project, and please post pics for us when you complete them.

Luvs