I have had good luck with Bar Keepers Friend on a rag and bucket of water, others have reported good results with TSP, water and a little soap in a bucket. I have used TSP in a bucket of water on my Scamp
with good results as a final wash before waxing.
Big soft scrub brush on a short handle proved very handy too.
I found the washing with rags and a brush, followed by a power wash rinse to be less work than StarBrite fiberglass clean/restore and wax product that I had to rub on and rub off. But I was just clean at the end of my process, not waxed. I used Red Maxx Pro #3 liquid for my wax job as a follow on process.