osmotic blisters

[paul oman]

Does anyone have a clue about how long it takes for hull blisters to
form? Has anyone heard of them forming on a new boat in less than a
year? I have always suspected they take a good long while to form, but
have nothing to back that up.

your thoughts please.

Paul Oman
progressive epoxy polymers

[mmd]

WoodenBoat's sister publication, Professional Boatbuilder, has done several articles on this topic. It might be worth your while to pop over to their website (via this website's links on the home page) and peruse the back issues listings. A call to Bruce Pfund at Bruce Pfund/Special Projects LLC might also prove to be a good idea.

[kc8pql]

I know of a Maxum power cruiser that blistered in one season in fresh water (about 4 months) It had been painted with epoxy barrier coat primer and anti-fouling except for where the stands were in the way. When the stands were moved only anti-fouling was applied in those spots and that's where the blisters formed. Ain't fiberglass wonderfull?

[Dan McCosh]

There is still a lot of controversy about the cause of blisters. One theory is that it is the result of uncured, mismixed resin, which means that they could occur very quickly, the notion that they are the result of water penetration implies they should take a while.

[Tom Lathrop]

Paul,

I'm a bit surprised that you ask this question with your experience with resin systems.

Blisters result from the interaction of water molecules and the weakness of molecular bonds between the resin components. Polyester resin molecular bonds are much weaker than epoxy bonds and are broken by the water vapor (solvent) more easily. Once a bond is broken (or maybe it was never properly formed), the natural reaction is to equalize the water purity on both sides of the membrane. Since the chemistry of the liquid in the interior is much more concentrated than the water outside, water moves to the interior in an attempt to equalize the concentration. Pressure increases in the area and a blister forms. Some blisters have enough pressure built up to spit at you when they are punctured.

The time taken for osmotic reaction to take place depends on the molecular bonding strength of the particular resin, temperature and the purity of the surrounding water. Fresh water is worse than water containing salt and other impurities. I've seen blisters formed on a FG boat of mine kept on a trailer where the carpet bunks contact the hull, apparently from rainwater. Also under flexible rubber water tanks on the interior of a hull.

Lower quality of resin and poor mixing or curing speed up the osmotic process also.

Some of this is from personal experience, some from studying "expert" reports and some may be hearsay.

Wood rocks!!

[Bob Adams]

Blistering? What is this blistering? My boat is 38 years old and has nary a blister tongue.gif

[ 11-07-2004, 11:12 PM: Message edited by: Bob Adams ]

[RonW]

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

Scroll down to blisters again...
wood rocks.....