Often when cleaning our boat, we have to remove stubborn stains and marks from the fiberglass. My girlfriend has discovered that Mr. Clean Magic Eraser, a common household product, will remove the toughest marks with little or no scrubbing.
I don’t know what the active ingredient is, but these cleaning sponges can remove black scuff marks and many other tough stains. No harsh chemicals are involved, so we need not wear rubber gloves, and there is no harm done to our fiberglass gelcoat. We have used the Magic Eraser for over a year, and our boat has never looked better!