You need either fiberglass cleaner,comes in a blue vaseline size jar and is a gel.Dont recall the name but it is 3 letters maybe FSR .That should work but since they are old stains you will need On-Off made by Marykate which is an acid so you probably want to avoid getting it on the pvc/hypalon part of the inflatable.If you are going that route you can also go on the cheap and buy toilet bowl cleaner which is practically the same stuff as On-Off but you can get it in the dollar store for a buck.Charlie