Try the following, at your own risk as they say. Splash some vinegar on a small part of the stain. Leave it 20 mins and wipe it. If stain goes then you know what to do if it lightens try repeating it. If this works you might want to try citric acid instead- it doesn't smell! If vinegar doesn't work try a good car polish- this puts a coat on the surface. If the surface is rough the top coat might have gone, there is a gel cleaner available at marinas that might help.