On my topside there are numerous round black stains (size of a 10p coin), particularly on the anti slip areas. Has anyone any advice how I can remove them?
Depends on what has caused the stains, but I've always found Starbright Hull cleaner to be good. It contains oxalic acid, which is probably the active ingrediant in it. Works well for me. About a tenner for half a litre, lasts a while though.
If its mildew, then try using 50/50 solution of Milton sterilising fluid. Worked wonders on a couple of kids lifejackets that weren't quiet dry when I put them away.