It depends on the hull
construction, level of damage and the quality of the repair.
is the resin being disolved by water
. A blister is the result of a chemical reaction between resin and water
. Simplified water passes through a screen
that is gel coat it mixes with resin and molecules expand but the screen
it now to small to allow the molecules to permeate and the pressure from the reaction causes the gel coat to bubble. Osmosis can be present with out blisters. If the screen is large enough to allow the expansion with out development of pressure. Often you will hear my old boat never had blisters. Correct but it probably had osmosis.This can't be good. also it may not be that bad.
Old hulls that were built before the limitations or possabilities were understood have glass and resin at levels that may be assumed sacrificial. As the cost of resin increased and understanding improved boats became lighter and resin saturation levels decreased. Some resins were developed to have different properties. One such resin was fire reatdent but also was very soluable. lightly built hulls with reduced resin ratios, cored hulls, poor layup
could have and I have seen significant weekening of the structure.
A superficial repair without proper removal
or drying of the hull has limited benefit. If you are really concerned you should get a surveyor
that understand the lay up schedule of your boat and can analyze the extent of damage and rate of deteriation. The inspection
should included a density test and the test should be performed at layers until a suitable reading is obtained moisture tests are good as are visible inspection
of the laminate condition.
Hope this helps