Just to touch on a couple points here. Blistering does not occur on all boats. There are several forms of blistering and it is important to understand which form a boat has.
The most common form of blistering occurred on US built boats constructed after 1972 and up through the mid to late 1980's as a result of the reformulation of polyester resins that occurred as the result of the oil
crisises of the 1970's. This type of blistering can generally be treated by barrier coating the below the waterline hull and the bilges of the boat. This form of blistering usually produces comparatively large diameter blisters that will progress through the laminate if left unrepaired. Grinding should be minimized on this form of blisters since the heat can produce blisters deeper within the laminate. Peels and sandblasting are a better course of action followed by a lay-up of F.G. cloth and either epoxy or vinylester.
The second form of blistering tends to occur boats built before the 1970'e reformulation. It typically is the result of over accelerated resins and resin rich laminates. This form of blistering rarely crosses more than one laminate lay-up and can occur as localized patches. In the worst case, where the boat was laid up with over catalized resin or over accelerated resins the problems can move quickly deeper into other laminate layers. Early Columbias and Coronados are considered to frequently have this problem. Many Oriental boats were prone to this problem long after the US had come to realize the problems with using over accelerated resins and resin rich laminates.
A lot of later boats have simple gelcoat
to laminate blisters. This occurs when the Gelcoat
goes off or gets contaminated before the laminate gets applied. This is sometimes called a 'friday boat' syndrome meaning that it is assumed that the gelcoat was applied on a friday and the layup
occured the next week. This is a pretty benign form of blistering that can be repaired by popping off loose sections of gelcoat with a sharp chissel, lightly sanding
the laminate and filling with thickened epoxy. This form of blisters are generally, shallow, small in diameter and appear more densely on horizontal surfaces.
There was mention of fire proofing of fiberglass
. That was not a code requirement nor an industry wide problem. Only very few manufacturers experimented with a fire resistant polyester resin, Valiant being the best known. It was not just the gelcoat that was fire-resistant resin. In many cases fire resistant resins were used throughout. This is a very serious form of blistering since it can effect the entire structure of the boat and there really is no good way to permanently fix this problem.
There was also mention that blisters were more of a problem with inexpensive boats. Strictly speaking that is not the case. Price
seemed to be less of a determinant that the materials and construction methods chosen. Valiants for example were a very high priced boat for their day. Late 1970's era Hinckleys have been known to have very serious blistering problems.
Higher quality thinner hulls were less prone to blistering problems since greater care in the lay-up was required and resin rich layups were avoided. Cored hulls tend to be less prone to blister problems and are generally easier to repair if a problem occurs.
Otherwise, if you only have a few shallow blisters, open them up, clean the area well, allow to dry out and fill with thickened epoxy, check the boat regularly and then move on in life.