Well-known member
12 Sep 2001
Home - Southampton, Boat - Gosport
I friend has just had his Westerly Chieftain (Centaur with an aft cabin) surveyed, and the report states that a moisture check was carried out using a Sovereign Moisture Meter calibrated for use on GRP on a scale of 1 to 25. Readings of between 18 and 24 were found on the hull bottom, and “total saturation” in way of the keel studs.

There were a dozen or so small blisters between the keels, none bigger than a 50p piece, and when sanding the gel coat, a few moist areas appeared. In every case, these could be ground back to visibly dry without going below the top of the mat layup.

The surveyor is suggesting peeling the gel coat and a full osmosis treatment, but that sounds a bit excessive, especially as it would probably cost a significant proportion of the value of the boat. We would prefer to fill the craters left by cutting out the blisters with epoxy and Gelshield the hull. Any experts out there with useful advice?


New member
4 Jul 2001
Australia, East coast.
Hi Stemar
It sounds like your gel coat has failed, this may well lead to osmosis, however this is not the end of your boating.
Your on the right track, grind out the moist areas and fill with epoxy resin and glass as required, then sand and apply 3 coats of resin to seal it.
I did this for a good few years; repairing half a dozed blisters each time I came out to apply new antifouling. However I finally decided the gel coat was no longer providing the protection it should and ground the lot off
After filling with unidirectional glass and epoxy resin, followed by 3 coats of epoxy resin, a couple of coats of International Interprotect and then polyurethane paint she was done, that was a few years ago and not a blister since.
Old Salt Oz


the surveyor is giving your friend the cautious advice which is to deal with the problem now before it gets worse. he cannot take the risk of predicting whether or when the deterioration of the laminate will affect the structural strength of the boat.
having had 2 osmosis jobs done myself, i would not hesitate to take a diy approach on a boat where the price of the boat did not warrant a yard job. you can do an osmosis treatment easily as well as the yard (you will take more care) and you will only lose out when it comes to resale, if then.
wouldn't recommend the approach suggested however. epoxy coating a hull with high moisture will only trap in the moisture, make the problem worse as long as the epoxy sticks to the laminate (which wont be long in practise).if the boat were mine, i would ignore it until i felt it might be getting too bad (but before it affects the structure) then do a diy job. get one of the mobile peelers or gritblasters in, wash repeatedly over some months until the hull dries to 5 or less on the meter, and then epoxy coat to the manufacturers spec.