it should last on plastic as long as it was done right and the panels are not damaged while on the road. the paint may be cracking due to not having a flex additive mixed in. I know some automotive paints come with it, such as sherwin-williams. When I was going to wyotech, the sherwin rep came out with a piece of a shower curtain sprayed with red paint. You could roll it up into a little ball, or crumble it as tight as you could and the paint would not flake off at all.