Wonder if it could be that the paints reacts differently on a plastic surface. Have you thought about Krylon paint, it is made specially for plastic surfaces.

It has to be clear, though. Clear, but colored, like stained glass. I Have used the Krylon for plastics, but I think it only comes in opaque colors.

When this "paint" came out the instructions were to use it from the bottle tip. and fill in the spaces that you had created with fake lead. You used to just take the top off and there was a tip on the bottle. To use you pressed the tip against the surface and pushed/pulled it across the surface to fill it in.Then you used the tip to 'smoosh' it all together nice and even. Be aware that this stuff doesn't hold up outside or in direct light. Fades fast. But for what it was made for, it worked great. Hope this helps.

I did some roses for mom's door glass between garage & patio which is covered so it gets daylight but no direct sun. I used a toothpick to fill in between the leading & also to get out the bubbles. You probably need to do the area several times, best if you can take shade off & have area as flat as possible so less "running of paint". I just squeezed a little out & then moved it around with toothpick or tip of bottle, toothpick works better in small areas & corners. Good Luck! Jan

What I don't have is a flat surface, though. That's why I'm afraid it will run into the low spots. I would have to brush it on. With enough coats, does the uneveness of the coat even out?

Never used a brush with it, I followed the directions in kit I was given. it says you can do the painting after you get leading done on a door window or whatever. Someone did a shower door, might have been shown over on GJ or Stained Glass & Mosiacs forums. She did the entire door & they are heavy so doubt she laid it down unless she did whole thing in a garage or something. I redid a couple of areas that I didn't think matched up with the rest of the roses & leaves while it was on the glass & I didn't have any trouble. Jan

It is not going to work well with a brush. It is best used in thick layers or a bead line and dropped onto the surface from the opening in the tip.

I like it for various things including mixed media like water color and using this for added depth or to give a painting a leaded glass effect. I made some very cute peel and stick lady bugs to go on my cards with it too.

I have used it for dots on handpainted greeting cards too- sadly the dots never fully harden and stick to the next card, even a year later they seem a bit sticky.

Oh speaking of sticky- it makes a nice window cling- we made them and stuck them to the microwave.

If you're using it on a textured surface, then yes, you will have thick and thin spots. Is the reverse side of the plastic smooth?

No, it's very thin plastic: so both sides are about the same, with the waffle texture.