HERE is a 1987 USCG report on cause/remediation of boat blisters. It's dry and difficult reading. Below is the summary section. Buried in #11 is likely the root of the issue - I could not find enough brain cells to read the entire report to see if this was adequately addressed.
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Listed below are the major findings of the American Boat Builders and
Repairers Association/University of Rhode Island research project. The reader
is urged to consider the implications only after a thoughtful reading of the
text.
1. Currently used glass reinforced polyester laminates are excellent
materials for boat building, but under some circumstances, can
exhibit blisters.
2. All resins contain some water soluble material (WSM). Only if
that material is concentrated at a point in the hull will it cause
blistering.
3. All blisters are caused by water diffusing into the hull and reacting
with water soluble material to form a droplet of solution which,
because of osmotic pressure, grows in volume and creates a force
which results in a blister.
4. Three types of blisters are discussed. Water soluble material concentration blisters are formed from clusters of materials present
in the hull when the boat is built. Stress induced blisters form
because water soluble material is concentrated by water diffusion
and stresses. Long term blisters form because of ester hydrolysis
of the polyester molecules.
5. Other factors being equal, gel coat thickness determines when
blisters will begin to form.
6. Laminate lay-up and post-cure temperatures did not have a major
effect on initiation time or severity for temperatures between
50°F and 90 0F. Higher temperature post cure should improve
laminate stability. The degree of cross-linking is a critical parameter
in determining the properties of a resin.
7. Design of the resin-rich region between the gel coat and laminate
is important in minimizing blisters.
8. Control -of air inhibition during lay-up can improve blister resistance.
9. Microscopic examination of blisters indicated presence of sawdust, disk cracks, promoter and internal stresses in the blister region.
Without such examination it is impossible to pin point the exact
cause of blistering in a particular boat.
10. Leaching of material from gel coats will cause surface crazing
of the material.
11. The blister initiation time and the severity of the blisters formed
during this study was a function of both the laminating resin and
the gel coat used. However, the area affected by the blisters was
similar in all cases.
The size of the blisters depended on the laminating resin used. The chemistry of the materials are complex and
variations were seen among generic types from different
manufacturers.
12. It is recommended that boat manufacturers institute programs
of quality control and quality assurance, with specification for
their supplies, to minimize the blister problem.
