GRP Osmotic Blisters

[Steve Smith]

FIBERGLASS HULL BLISTERS

Back in the seventies I discovered what exactly caused blisters in Fiberglass [meaning polyester-resin/fiberglass, also known as GRP, short for Glass-Reinforced Polyester] hulls, and invented the Barrier-coat technology that is now widely used. Here's a How-To-Fix-It Application Note that I wrote about that. This is the short version, with none of the explanations of why or how or interesting details. This is just the brief repair instructions:

Does your fiberglass hull have blisters? If not, you may be lucky. Perhaps they will develop, perhaps not. If blisters develop and are ignored, your hull will develop major structural damage. I believe blistering of fiberglass hulls can be prevented for decades, depending on hull condition and repair workmanship.

I developed the Barrier Coat Technology in 1975 and hulls correctly done since that time have shown no further damage.

The essence of the problem is that water permeates the resin system and then causes what damage it can because polyester resin and its additives inherently decompose in water.

If one can put a totally impermeable barrier between the water and the hull, then the damage could not occur, regardless of how susceptible the resin of the laminate might be.

There is no such thing as a totally impermeable moisture barrier, but the U.S. Navy has, over the years developed a great deal of coating systems technology. Further improvements by Smith & Co. have led to our High-Build Epoxy Paint. Its remarkable moisture resistance comes partly from the blend of mineral fillers used in the system, which increase the diffusion path length to several times the coating thickness, and partly from the unusually dense and water-repellent molecular structure of the resin system. This reduces the concentration of water in the hull. Interior ventilation to move out of the below-the-waterline area everywhere the inevitable moisture that does get in, is the second half of Blister Repair. General external hull repair instructions are as follows:

1. Grind out or sandblast all blisters down to sound laminate. Get under all of the loose material. Some blisters may have a colored liquid in them. Don't get any of this in your eyes or on your skin - it may be very caustic. Some hulls may have an advanced state of chemical decomposition. This may be recognized by a strong chemical smell such as vinegar or a solvent, which may be noticeable after peeling the gelcoat or grinding down the hull. An advanced state of chemical decomposition may also be recognized by liquid, apparently water, oozing out of the laminate where a blister has been ground out, and leaving a visible residue which dries on the surface. Such hulls should have the entire gelcoat ground off or peeled to expose the underlying laminate containing chemical decomposition products. This may also require removal of some laminate. Chemical cleaning may then be accomplished in any of several ways: the hull may be hosed down with water every hour for a few days; a garden hose with many small holes such as is used to slowly water a garden may be draped around the hull to slowly and continuously rinse the hull with water; or the hull may be launched and left in the water for a few days to a week and then hauled out again. Some fresh water rinsing and a week or three to dry out (depending on weather and temperature) should then give a hull sufficiently dry to continue. Simple moving air from fans evaporates water more efficiently than complex dehumidifiers, etc.

2. Sand any gelcoat that is left. You must get past the gloss surface of the gelcoat. It is a wax-rich surface, and it may also be degraded by exposure to the water. With many large blisters, it will be necessary to strip off the gelcoat and treat as discussed above.

3. Allow a week or two in warm dry weather for the excess water to evaporate out of the hull.

4. Apply Clear Penetrating Epoxy Sealer to saturate the laminate. Allow a few days to perhaps a week dry and cure; then apply a second coat of which little should soak in. In extreme cases a third or even fourth application may be necessary. Allow sufficient time between each application that the hull no longer smells strongly of solvents before continuing; this may take a few days to a week in moderate weather. In cool to cold weather the Cold Weather Version of our Clear Penetrating Epoxy Sealer should be used.

5. Apply two roller coats of High-Build Epoxy Paint.. Use the red for the first coat and blue or green for succeeding coats. Allow 4 hours drying time between coats. Allow a day or two for the epoxy paint to fully cure so it can be sanded without dusting.

6. Fill all holes with Fill-It Epoxy Filler. Allow to cure overnight, then sand. Stop sanding when you expose the red layer.

7. Apply three more coats of High-Build Epoxy Paint. 4 mil wet film thickness per coat is equivalent to 400 sq. ft/gal/coat.

8. Several days further drying time should be allowed to reduce the possibility of solvent blistering. The Antifouling Paint may then be applied below the waterline, and topcoat and bootstripe above.

And there you have it. It’s a fair amount of work, but it’s straightforward. Beware of yards that haul you out and want to sell very exotic, bizarre and expensive processes. I have yet to see these be necessary, although many are sufficient, IF INTERIOR VENTILATION ISSUES ARE ALSO ADDRESSSED.

[SKYCHENEY]

Or...as Pascoe says, with hulls this thick, just forget about it. Those blisters will never cause any problem, they are just cosmetic.

[Steve Smith]

It depends on the hull, and the workmanship. Some have gone to almost runaway decomposition of the laminate, as Pascoe has discussed in glowing detail.

A Hatteras is surely of better manufacturing quality than some I have known, but I would like to hear from owners of older Haterii, as to what their experiences have been.

[MikeP]

There is NO history of blisters on old Hatts being anything but cosmetic - a good thing for a buyer to demand a price reduction but as far as a real concern for hull integrity, it's a non-starter on a Hatt.

They are not Searays or whatever, as Pascoe makes clear and repairing them may be a good thing from a resale appearance viewpoint when selling to folks who are not aware of the specifics but otherwise it's a total waste of time/money.

[Finalee]

Have not heard of ANY structural problems on Hatts from blisters...

[spindrift]

OK one of the problems that occurs on the internet is that we need to be careful about information that we put on the site. I have been in the Marine Finance business, have owned and operated crewed and bareboat charter boats, and am an absolute stickler on having all my work done at Class A yards such as Hinckley, Wayfarer, or Rybovich Spencer. I also only deal with class A surveyors. I take the opinion of Anthony (Tony) Knowles who runs Newport Marine Survey as near gospel on the matter of Osmosis. Three of the 10 boats I purchased over the past 10 years have had osmosis; we fixed it properly one Job at Palmer Johnson, the other two at Hinckley. In all cases, the problem was cured, and did not reappear. Tony has advised me on the issue of older Hatts and proffers the following advice. If not addressed, Osmosis will continue, it may become a structural issue (it did on one large sail yacht I owned), and will not cure (no pun intended) itself. Tony has said in these days of high fuel prices, a rough hull that has extensive defects, blisters, and gouges, will increase drag, and fuel usage.

In short, I have rejected one Hatt over the past 6 months because of osmosis because we were only going to buy it to add to our charter business. In order to conduct a proper osmosis treatment, whether you use the method outlined above or use a first class yard experienced in this type of repair it takes a lot of time...months to do the treatment... not done over the week. There is an outstanding article on this in Power and Motoryacht this past spring.. I think the May issue.

Now you can say no big deal. Many of the local surveyors in Florida may also say no big deal. But when one of the most extensively read powerboat magazines devotes several pages in an issue on osmosis the issue becomes a concern. If you have a problem with osmosis it is far better for you as an owner to fix it over the winter layup than to expect a buyer to do this in the spring.