For gosh sakes, don't spend more than the boat is worth trying to repair
blisters.

Have a qualified surveyor look at the hull once it is hauled out and the green
stuff power washed away. If there is delamination, you will need to make
repairs. If there is nothing more serious than garden variety gel coat
blisters, yes, you technically have a "defect" but one that is never going to
effect the structural integrity of the boat. A gel coat blister is like a mole
on the butt of a beautiful woman- it "shouldn't" be there, but it does nothing
to detract from the overall enjoyment or structural integrity .

If you want to address the blister problem on a permanent basis, you will need
to peel away all the existing gelcoat and the top layer or two of roving. Then
get the hull as dry as possible. Finally, replace the removed layers with
vinylester, rather than polyester. On an old boat, it ain't worth it.
There is a huge failure rate when lesser methods are employed.

Biggest effect that blisters have on a boat is their ability to depress the
price when buying or selling. Funny how many guys who insist on a $10,000-
$30,000 discount to accomodate "blister repair" put the exact same boat back on
the market several years down the line with the exact same, untouched,
blisters. :-)

Typical conversation at resale time?
Current buyer: Omigod! Blisters! What kind of crap are you tryng to pawn off on
me? I need a huge discount! I won't even be able to use the boat for months,
while it sits in a shed someplace, at $30 a layday, drying out!

Current seller (who gave the exact speech outlined above when he bought the
boat):
Oh, fiddlesticks. Purely cosmetic. All those bllsters were there when I bought
the boat. Never gave me a moment's problem.

One of the portions of Dave Pascoe's site that makes good sense to me is his
article
about the considerations involved when acquiring a boat with blisters. Here's a
link.

http://www.yachtsurvey.com/BuyingBlisterBoat.htm