Martin one thing that used to work in loo's and would shift anything including the skin from you hands
, although not sure if legal anymore and hardly high tech
is spirit of salts, used to get it from chemists.
Always wondered if it damaged the surface, but we used it on one loo which was growing a mushroom farm from it and reviewed it again a year later with no probs.
This should be a last resort though, as used incorrectly it can be extremely harmful.
Oh for the days before legislation when health and safety was merely a concept and playing with dangerous chemicals was a lot more fun
disclaimer: Im kidding!
DP