Sailboat Repair: How To Fix A Blistered Hull

How to repair a blistered hull before it becomes a larger problem.

Sailboat Repair: Fixing a blistered hull

Older fiberglass sailboats that are kept in the water full-time are often afflicted with what is known as the "boat pox" or hull blistering. This phenomenon affects many boats, but inexplicably, does not occur in all boats of the same age or even made by the same manufacturer. The reason for it is thought to be improper mixing of the resin during construction, or simply bad batches of resin, but most boat repair experts can't agree on the cause. The cure is also hotly debated, but it is generally accepted that hull blistering can be fixed, if not permanently, at least well enough to last until the next time the boat is hauled out for bottom painting. One thing is for certain, the problem has to be addressed, otherwise your boat hull will keep on absorbing water through the blistered areas until the hull becomes saturated and is in danger of delaminating.

The first step to repairing a blistered boat hull is to haul the boat out and thoroughly clean the bottom by removing any marine growth. With the bottom clean, you can identify blisters, which may range in size from the diameter of a half-dollar to tiny bubbles the size of a pencil eraser or smaller. Normally, these blisters only affect the Gelcoat, which is the outer, pigmented coat of resin which gives your fiberglass boat its color. There may only be a few blisters, or there may be several hundred. In extreme cases, you will have to remove all the Gelcoat from the hull below the waterline, which is a huge and time-consuming task. Most blistering problems are not so severe, however, and the hull can be spot repaired by addressing each blister individually.

When the blisters are popped by piercing them with a sharp knife, a brownish liquid that smells like vinegar will ooze out. This is water mixed with the fiberglass resin. All the blisters must be opened to allow this liquid to drain out. Use a grinder or powerful electric sander to remove all the soft areas of paint and saturated fiberglass around the blister, but be careful not to grind all the way through the hull or into the dry layers of glass cloth under the blister.

This done, you must then let the hull dry completely out. This requires patience, as the process could take several weeks or even a few months. It's important though, because if you repair the blisters with saturated water still within the hull, they'll simply come right back to haunt you.

When the hull is dry and all the blistered areas are showing no more signs of oozing liquid, you are now ready to fill the holes you ground away and smooth the hull for repainting. Use a boatbuilding epoxy resin, carefully following the manufacturer's instructions for mixing it up to the proper ratio. Paint on a coat of clear resin in all the damaged areas and let it cure, then go back and mix a thicker batch of epoxy and a fiberglass filler compound such as colloidal silica or microballons. This thickened epoxy should be mixed in batches about the consistency of peanut butter, so it can be smeared onto the repair areas with a putty knife. When it cures, the thickened epoxy can be sanded to fair the surface in with the surrounding hull. Additional applications may be necessary for larger blisters and the larger cavities you had to grind out to repair them.

When all the blister holes are filled and the hull has been sanded and faired smooth, put another coat of clear epoxy on over the repairs and when this cures, you will be ready to paint the bottom of the boat with whatever type of anti-fouling bottom paint you use for your sailing area.

After the blister repair is done and the bottom is painted and your boat is back in the water, try to check it frequently by diving under the hull with a mask and snorkel to make sure blisters are not returning. If you did the job right, you shouldn't have to worry about blisters again until your next scheduled haul-out

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