Blisters and Osmosis

While cruising on Callipygia, with a fiberglass hull, we periodically hauled her out for repairs and to check the condition of her bottom. Here's what we learned about blisters and osmosis.

          We had cruising friends who had to do major bottom repairs, and heard of others whose boats been condemned because of hull problems. But we learned that osmosis is both an inevitable and a manageable condition, and definitely not a cause for panic. By its very nature, FRP (fiberglass) sitting in water will absorb it--after which many things may happen, usually very slowly. For the most part this can be taken care of with regular maintenance so that the life of the hull can be extended for a long time. This article summarizes our research on the causes and cures of this problem.

          But first we'll report on the "problem" we found when we got hauled in Trinidad (August, 2003). The cream-colored Awlgrip (which had been painted on top of the original dark blue topsides gelcoat 3 years ago prior to our purchasing Callipygia) had some little blisters along the waterline, and the bottom paint had lots of little red blisters in and around the bilges. We scraped some of the blisters on the bottom paint and water came out--and in some cases a kind of ooze. We went into a bit of a panic until we did more research. Then we learned that there's a divergence of opinion as to appropriate remedies, but often the drastic treatments (removing and replacing fiberglass) do more damage than good.

          We consulted with surveyor Billy Wray, who said most of our boat's blisters were basically between the gelcoat and the primer or bottom paint, though some maybe went on in through the gel coat. As far as the Awlgrip blisters on the topsides, those seemed to be just in the paint where it was continually lapping in the waves. We had put bottom paint over the old bootstripe at the last haulout because the old bootstripe had become largely submerged as we loaded up the boat with all our liveaboard belongings. Billy's recommended treatment for the Awlgrip was to sand those blisters down and put on a new bootstrip to protect the waterline. For the bottom his recommendation was as follows:

    1. Strip all the antifouling paint off.
    2. Knock the tops off any blisters that still remain so they will drain.
    3. Wash the hull thoroughly with a mixture of vinegar and water, then a mixture of detergent and water.
    4. Let the boat dry at least 2-3 weeks.
    5. Test with a moisture meter. If it still seems to be moist, let it dry a bit longer.
    6. Grind out and fill any remaining blisters with an epoxy/silica mix (West System).
    7. Fair the surface.
    8. Everywhere there were blisters, put on 5 coats of good quality clear expoxy, then 2 coats of epoxy primer.
    9. Put on 2 coats of antifouling paint.
    10. Put a new boot stripe on to cover the area where the Awlgrip blistered because it was at the waterline.

          We and assorted boatyard experts then did a fair amount of diagnostic scraping, and we learned that on top of the gelcoat (lite blue), there was a barrier coat (black), a coating of primer (white), and many layers of bottom paint (first green, then red). After the old bottom paint was scraped off, it looked like the old barrier coat was failing, so we had that all scraped off too.  We let her sit for several weeks to dry. Then we had 10 layers of epoxy applied to the entire bottom, some primer, and finally new bottom paint. We now had a dry boat, almost good as new.


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Bill Dillon (KG4QFM)
Pat Watt (KG4QFQ)
This page was last modified on August 9, 2009

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