After reading this whole thread, it's my opinion after painting bikes for 40 years, is to just grab some sandpaper and go to it. Nobody likes to sand, but with plastic parts, using any chemicals is bad. It may not look like it's soaked in or contaminated the plastic, but it's so much better to be safe then sorry. Most people are afraid to start with an aggressive sandpaper. It's your best friend and saves a lot of time and energy. Start with 80 grit if it's 20 coats of spray can paint on it. At the very least you can start with 180 grit and maybe finish with 320. You need a little tooth on the parts for the primer to adhere to. I've had great success with polyurethane high-build primers. No flex agent need. Block sand it with 400 grit, spray a couple more coats of primer, block it with 600 and spray your base coat.