Yep, sandpaper helps. Using high-grit sandpaper I was able to buff the starboard side of my boat, removing at least some of the weird
paint-like substance that PO used to cover up "dock rash". Dock scratches themselves were really not that bad, and if PO only used the same method - the boat would have looked good as new, but too late for that now. As it stands, proper sanding can achieve quite a bit even on essentially damaged gelcoat.