Brown Resin Droplets on 1976 Hull

Repair or modification of Boston Whaler boats, their engines, trailers, and gear
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Joined: Sat Nov 25, 2023 2:13 pm

Brown Resin Droplets on 1976 Hull

Postby Southporter » Sat Nov 25, 2023 2:37 pm

On a very well used 1976 McKee hull [whose construction is similar to a Boston Whaler Unibond hull] that has been in my family for three generations there are small brown resin droplets appearing on the hull while it is just sitting under the boat shed. I do not believe this hull is water logged, as it sits high in the water, and the hull has been kept on a trailer its whole life.

Q1: how can weeping resin be cured or prevented?

Some recommend vacuum bagging the hull, but even with that, it could take years to fully dry. I'm not looking to dump thousands into a boat that will never be worth it. Just looking to enjoy what I have.

Q2: should I just ignore the weeping resin spots and continue to use the boat?

Q3: should I redo the entire gel coat?

Q4: should [a protective barrier coat be applied to the hull] bottom?

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Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula

Re: Brown Resin Droplets on 1976 Hull

Postby jimh » Sun Nov 26, 2023 9:03 am

The appearance of brown liquid resin would usually manifest only from breaches in the gel coat layer such as fastener holes that penetrated through the gel coat and laminate layers into the interior foam. The source of these brown resin droplets is thought to be from uncured polyester laminating resin that mixed with water vapor that condensed to liquid that was produced as part of the chemical reaction that created the foam from a liquid, or from water ingress due to poor sealing at the fastener hole.

Soft spots with brown liquid that appear on the hull where there was no drilled hole and in areas on the hull that are normally submerged when the boat is in the water are probably osmotic blisters caused by seawater passing through the gel coat layer and getting into the fiberglass laminates.

For advice on repair of osmotic blisters you can find many articles if you search with these arguments:
  • repair
  • boat
  • hull
  • osmotic
  • blisters

The appearance of osmotic blisters on fiberglass-reinforced plastic boats was quite a problem years ago. Improved resins used in molding hulls and the use of protective barrier coats on the hull surfaces that remain in the water have overcome the problem of osmotic blistering in better quality boats given proper care.

Boston Whaler has always advised owners to employ a protective barrier coat on their hulls if the hulls will be left in the water for more than a few days at a time. See ... tml#bottom

The general remedy for osmotic blisters is to burst the blister, allow it to dry, grind off the residue, and repair with new resin and gel coat. A hull with severe blistering may need substantial grinding, and perhaps application onto the entire hull of a completely new top coat, possibly a paint instead of a gel coat resin.

In the case of a 47-year-old small skiff, willingness to undertake the amount of labor and material costs that may be needed to effect a cure is difficult to judge by anyone other than you, as there may be sentimental value in the hull that warrants expensive repairs.

As long as the defects are just cosmetic and are not structural, the utility of the boat may remain unchanged, even as its appearance suffers.