Use Klean-Strip brand Fiberglass Paint Remover. I bought a gallon from an automotive paint store locally. It's formulated to strip paint, acrylic, polyurethane, lacquer, and enamel from fiberglass. Automotive body shops use it for corvettes and fiberglass tops.

I stripped a Composite ARF MiG15 with it, and it worked perfectly. Brush it on the surface, wait a little while for it to bubble the paint, then scrape it off with a putty knife and paper towels. Wear gloves, and wash the stripped parts with soapy water to remove any residue before painting.

Good Luck.

Regards, Les