Paint on Flex Paste

I recently used Woodland Scenics Flex Paste to “line” a sream bed in a rock formation. Now I find paint won’t stick to Flex paste.
Anyone have suggestions as to how I can paint this surface?

Thanks,
Dave

I’ve never used flex paste before. I had a similar problem with silicone one time though. I solved the problem by painting the silicon with a caot of white glue, then painted over that when it was dry.

Try a really good primer. The black primer made by teh Warhammer folks (found at your local game store) will stick to just about anything. Then the paint will stick to it. I would bet the Testors and others wold help as well.