A small population of Pinhead sized blisters are usually NEVER a
problem.
Osmotic blistering that occurs in the gelcoat layer or the
non-structural matting layer underneath the gelcoat are simply
"cosmetic". Only when the osmotic blistering begins to penetrate into
the structural layers of the laminate (roving or cloth) would one then
prudently consider a total restoration 'bottom-job'.

They will NEVER go away as what you are looking at is the cosmetic
surface resin has begun (just a teeny bit) to degrade - analagous to
rust in steel and iron.

Your alternatives a
1. open the blisters by grinding, etc. and simply fill and fair the
crater with thickened epoxy.
2. Let the boat dry out a bit and have a moisture analysis performed.
If the analysis moiisture indicates a high % of water absorbed into the
fiberglass, then consider to remove the matting layer and gelcoat (by
'peeling", and recoat with multicoats of epoxy barrier coats, etc. If
high moisture is found then consider to have a sample bored from the
hull and send it to an analysis laboratory to analyse for hydrolysis
and other forms of degradation - dont simply commit to the backbreaking
and super expensive job of rebuilding the bottom.

Simple answer: if you only have pinhead blisters, simply open them,
wash them out, fair in epoxy, etc.

If you applied a barrier coat 14 years ago, the recommended 'thickness'
of such barrier coats in recent years has essentially doubled. If you
didnt apply barrier coat and only have a few blisters in the
gelcoat.... wait until next season and just 'fill and fair' those that
you see.



In article , PETER KEATING
wrote:

I wonder if anyone can give me some advice. I have been cleaning off the
bottom of my 14Yr old Nimbus motor cruiser and I have noticed some pinhead
size blisters on the underside. My questions are these (1) will they go away
over the winter by drying out. (2) will they dry out if I use a hot air gun
on them, if so I can then Epoxy the hull.
Your assistance will be very much appreciated.
Thanks. Peter