I remember watching one of Ospreys staff remove a moulding burr caused by using a two piece mould with a spoke shave. After polishing you couldn't see a mark, however he had been working in GRP for many years.
High pressure water just removes the tops, leaving the small round foot behind, although its good for the weed and slim. Think the barnacles actually secrete a sort of cement to fix themselves on rocks or a boat.
How about jewellers rouge, its like "t cut" but made of a diamond paste. Alternatively two coats of antifouling and dont' worry about the problem again.
Pete (not the one with the speed boat)