[CheapPelican]

I asked one of the guys in the paint department at work and he said that "If the paint is lacquer based, like krylon, use lacquer thinner. Put a couple drops on there and if it is lacquer based, the paint will blister and peel almost immediately, then you can scrape or rub it off with a rag. Use a wet rag to immediately remove any remaining lacquer. Afterwards test the stripped area by scraping it with a screwdriver or nail. If it is not soft or does not scratch easily you should be good."

They use lacquer thinner to remove graffiti from playground slides which could be, or at least is similar to, polyethylene.

Disclaimer: I have no experience with this so do this @ your own risk