They are made of polyethylene, a version of the same stuff most jugs chemicals are sold in are made of, meaning that most chemicals won't affect the plastic, even the nasty stuff, which is also why paint doesn't stick to it well. So, use lacquer thinner to remove paint, use a lot of it and use a non abrasive pad or cloth to wipe it off. If that fails try harsher chemicals, Acetone, even paint stripper, just use small amounts of the harsher stuff in a test area to be sure it won't melt the plastic, it shouldn't.