In July we bought a 93 C30. While being surveyed one blister was found. See, just one year out of the 5 year warranty, so there is really no protection from them. I have heard that mid-80s C-30s had a pretty common blister porblem. Who knows. If they are light and can be filled, do so. You say the boat has been on the hill for a while so I'm sure the moisture level is very low. You can always peel it and put on an epoxy barrier coat. That should last years...