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Por15 has a paint stripper but I would be very careful about getting it on any bare fiberglass. Chemicals are very hard to remove once they are absorbed into the fiberglass and painting over that can be problematic.
I don't think you can strip those bumper covers. The stripper will atack the urethane. I think you have to sand some of the paint off.
The Por15 stuff I used many years ago claimed it would not hurt plastics or rubbers of that type, and because I have a '77 I even called and verified it was safe to use with one of their techs.
Also used Bulldog stripper. Purchased it at our local Auto Paint store.
Takes about 3 to 4 cans per bumper, depending on how many layers
of paint. Use plastic body filler applicators to remove the
loosened paint instead of a metal scraper so the bumper is not
gouged.