I have repaired literally thousand's of blisters while working in a boatyard in the
Florida (USA) panhandle, and what follows will probably be much more than you want to know.
Small blisters the size of a dime are mostly cosmetic and produce no structural damage. Many times I repaired them because the owner was concerned with the cosmetic aspects, but one boat had over 300 the size of an orange, standing up almost 1/4". Some produced
delamination within the hull for a foot or more around them. So, blisters can be destructive to the integrity of the hull laminate, so much so that in extreme cases it is necessary to peel the outer layers of glass from the entire hull, and replace them with
epoxy laid-up new glass cloth. The issue is to stop blister growth, and to do that you need to know the mechanics of blister generation.
The following applies to polyester resin impregnated
fiberglass hulls, since the new vinylester and
epoxy resin/gel coats are themselves highly impervious to water.
Blisters are caused by inperfections within the hull lamination - either improperly cured resin, voids within the
layup of the laminations, and even the use of certain kinds of resin (fire-retardant resin is notorious, hence the 80's problem) Since it is virtually impossible to lay up a hull without imperfections, it is absolutely essential to maintain an epoxy barrier coat between the hull and the
anti-fouling paint. This, in itself, is the most effective preventative measure to avoid blisters.
The imperfections within the hull laminate create a chemical attraction to water. The water, at a molecular level, penetrates the gell coat (hence the Osmosis designation), and forms a new molecule with the resin. This new molecule (which smells like vinegar) is larger than the original water molecule, and thus is trapped within the laminate. As more water molecules are attracted through the gelcoat membrane, the pressure builds inside the laminate until the telltale blister appears.
First of all, blisters are evidence that the intergity of the barrier coat material has been compromised. The remedy for multiple small blisters is simply to remove the
anti-fouling paint, and apply a good epoxy barrier coat to the entire subsurface area. Absolutely follow the directions about the barrier coat thickness. Here is a place where more is better than less. Also follow the directions about drying time before the application of anti-fouling paint.
If there are a few isolated larger blisters, use a grinder and remove the blister until there is clean un-delaminated glass all around it. Be sure to grind deeply enough to remove all damaged
fiberglass. If the
depth is significantly a part of the hull thickness, cut multiple pieces of glass cloth, first small in diameter, and increasing until they are the diameter of the section ground away. Four or five pieces should be sufficient. Remove all paint (barrier coat and anti-fouling) from a small area around the blister
repair. Clean the area with acetone, paint the area with
epoxy resin. When the epoxy has become tacky, begin to lay in the glass pieces you cut, starting with the smallest. Be certain the each piece is saturated with epoxy resin and that there are no air pockets. Continue until the glass patch is at, or slightly above, the surrounding hull surface. Let the patch cure overnight, sand it smooth with the hull surface. If necessary use a mix of epoxy resin and colloidal silica to fair the
repair with the hull surface. Let it cure overnight. Then apply sufficient thickness of barrier coat and anti-fouling paint.
On small blisters, up to about the size of a quarter, you can uses a router set to 1/8"
depth, and probably remove all damaged glass. Then simply fill the repair area with the above mixture (epoxy/collodial silica), fair, and paint as above.
I have only anecdotal experience with boats kept in fresh water, but that evidence seems to indicate that fresh/salt has little to do with blister formation. Again, the best guard against blisters is a properly applied barrier coat of proper thickness.
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