...you are not EXTREMELY careful and wear protective EVERYTHING while doing this, OUT in the OPEN AIR.
Much easier, by FAR for me, is that wonderful trailer companion of mine, BARKEEPER's FRIEND. It's a cleanser that sells for under a dollar and is UNREAL in it's cleaning
fiberglass capabilities...of course, I cleaned old oxdized trailers and not pristine ones with perfect gel-coat finishes!!!!