TWL: RE: experience with total bottom job due to blisters (long)

Tue Nov 25 09:51:27 EST 2003

Having been in the boat business for a long time the blister issue has been
around since fibreglas one way or another. The curing process of fibreglas
resin and catalyst is an imperfect chemical reaction at best. You have to
have the exact ration of resin molecules to activator molecules or there
will be some of one or the other that didn't find a mate. When this happens
you have a lonesome bachelor or bachelorette that will forever be looking
for a mate. For most FRP, Fibreglass reinforce products, this is not an
issue. For FRP immersed in water it is. Water and water vapor will penetrate
most things given enough time...lets not get back in the "corrosive ion free
100 pure atomic distilled water :-)

When the water vapor gets thru the protective layer on a hull, whether it be
gelcoat or some other type of layer, one of these bachelors or "ettes" will
mix with the water molecule. Not a match made in heaven but better than
loneliness. One match isn't going to even be noticed in the great scheme of
things, however 100's, 1000's and millions will be do to that high school
thing you recall in Chemistry 101....osmosis.

When we have these strange bedfellows inside the hull in a solution it will
try to equalize itself and the easiest path is back thru the gelcoat. Bad
news however. The molecules of the new solutions are larger than the water
vapor molecules that came in and they can't get back out thru the same path
:-(

So what we see upon the hull are what we call blisters. Now a boat that
routinely is hauled for storage gets a chance for the hull to dry, a
relative term as nothing is ever really "dry". If a hull is left in the
water year after year it will continue to absorb more water vapor and find
more bachelors. Eventually, if left in long enough, all hulls will get
blisters, IMHO, as I have yet to see any brand that is free from them long
term. Whether you see them or they are an issue will depend on the closeness
the manufacturer got the original mix of resin and catalyst correct.

The repair methods out there on the market remind me of the types of
additives to make you get better mileage, or the spam on email that promises
to......

I have seen, tried and watched about all the methods known to man in
attempts to repair them. From all of this I have concluded that the overall
diagnosis and treatment is not a science but just old fashion common sense.

1. Never, never grind out a blister no matter how small or large. The
grinding process by its very nature drives the grinding residue into the
pores of the FRP. This does two things, one it prevents the "solution" from
evaporating or any flushing solution from getting in to clean the
solution/salts out and second it actually leaves more foreign substance
embedded in the hull when it is coated. This will just begin the process all
over at some future date.
2. Chisel out the blisters...this sounds like a destructive process but
doing this will leave all the raw fibreglas exposed and allow a much faster
and more complete drainage of all the fluids and salts. Naturally you only
chisel off the gel coat or a layer or two. It is very obvious when you are
at the bottom of the "pocket". If you have many small ones then a peeling
machine, which is nothing more than a modified planning machine whether
rotary or drum style. It too "chisels" the gel coat and up to several layers
in one pass. The depth is chosen to catch 90% or so of the pockets. The rest
need to be individually chiseled out. No sense it taking off more than
necessary ;-)
3. Wash the open fibreglas with fresh water! More pressure the better and
several times during the first couple of weeks. This really flushes the
solution and salts from the hull.
4. Then let it dry. How long and how dry is the mystery. Some like 10% or
less others will swear that 20% is OK. In any case once it is dry a good
flush with alcohol is worth while. Personally I use a garden sprayer and 10
gallons will do a 45 footer.
5. Now is the first time a grinder should see the hull. A 24 grit will dish
the edges and generally smooth the pockets. It also removes a gorser slag
which won't imbed in the GRP as easily. If it was done with a peeler and the
guy did a good job then a low viscosity slow curing epoxy rolled on will
soak into the raw glass. Several wet on wet coats may be required to make
sure you see no dry spots. Remember that you need to use a use a
non-blushing epoxy for this part. A bushing epoxy will leave a surface that
must be washed before sanding to remove every trace of the blush. If not the
again you will be embedding foreign substances in the hull waiting for
future water vapor to arrive :-(
6. Now you are ready begin the "closing up process. If you need to add
laminate back do so now and make sure what you are using is compatible with
the epoxy. Some mat and cloth has sizing in it. Again not good.
7. Now you should be at the "fill and fair" stage. Once you have this done
it's time to go for the "barrier coats". My preference is a high solids
non-blushing slow cure epoxy and 6-8 wet on wet coats. If rolled on and
tipped correctly you should have a run free nice looking very shiny hull
when done.
8. A non-sanding etch product may be applied and then the bottom paint of
your choice. The other option is to apply the first coat of bottom paint
before the last coat of epoxy goes off. Sanding is a poor third option.

Do one step wrong, sloppy or with the wrong type product and you might as
well not do it as you'll have "osmosis" back in the future. In the worst
case you might encapsulate some solution and if it can't relieve the
pressure by pushing on the new epoxy/laminate you applied it will push to
the inside :-(

Sorry this got so long but this procedure has worked for many years on boats
in tropical waters for years at a time without a failure unless the rules
weren't followed. The oldest full repair done this way is 12 years now and
not one new blister :-) YMMV

Dave
Swan Song
Roughwater 58
Tortola, BVI