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HomeCruiseWhat to Do About Boat Blisters... Welcome to Boat Pox 101

What to Do About Boat Blisters… Welcome to Boat Pox 101

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Boat blisters are a delicate subject especially when discussing how to repair them.

Unless you have a lot of time to dry out the boat and the money to cover the costs of peeling the hull, it seems repairing blisters is a black art. Some claim that if you only have a few blisters it’s best to leave them altogether, particularly from a resale point of view, as an un-tampered hull will be more attractive to a buyer then one with numerous repairs. If you are going to repair them then you need to consider how many blisters you have and whether it will affect the integrity of the hull.

When we bought Alianna, our Corbin 39, we knew that we had a blister problem and this was reflected in the price. We were then left with the decision on how we were going to tackle it. We knew nothing at the time and have learnt a lot along the way; including that there is still no 100% cure or boatyards would be offering lifetime guarantees when doing an osmosis job rather than a couple of years. As we were keen to go sailing and not wait for the hull to dry out, we choose to deal with the blisters individually. This is how we repaired the blisters in our thick cored hull. We do not claim that this is the right way for everyone, there are so many differing opinions, but this has worked for us and keeps us sailing.

Sail Cloth and Sail Durability

First you need to identify and open up the blisters.

This should be done fairly soon after you haul out as some blisters have a tendency to shrink if left for a while. On this occasion we decided to remove all our old paint, something we had never done before, but this is not necessary to repair the blisters. If leaving the antifouling on, wetting the surface helps in identifying the blisters.

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Wearing protective clothing and goggles, take a grinder with a 36 grit sanding disc to the blisters and carefully open them up. You will see the liquid seep out, keep grinding until you are through the damp patch and can reveal undamaged laminate. Feather the patch until it is about an inch larger in diameter and circular in shape as this makes filling easier. Repeat the process on remaining blisters. The open blisters will then need to be thoroughly scrubbed and washed out with fresh water to remove contaminants. Repeat the process several times – a pressure washer would be handy here. Leave the opened blisters for as long as possible but a minimum of several days.

Boat Buying Guide Boat Maintenance Costs

When you are ready to start repairing the boat blisters, first prepare your materials.

You will need:

  • good quality epoxy resin and hardener
  • fiberglass material in the form of six or eight-ounce cloth (chopped strand mat is considered non-compatible with epoxy resin)
  • colloidal silica
  • microballoon fillers
  • scissors
  • disposable gloves
  • a small paint brush
  • acetone
  • fiberglass roller
  • squeegee.

What is epoxy and does it matter what type of epoxy you get?

Cut the fiberglass cloth into circular shapes in varying sizes.

Start at just less than one inch in diameter, increasing the size to cover your largest blister repair. Clean all the hollows thoroughly with acetone and a clean rag or paper towel. Don’t attempt to repair too many blisters at once. If you have only shallow repairs to make then these can be filled with Colloidal Silica. Mix up a small amount of epoxy resin, add the filler until you have the consistency of peanut butter. Fill the blister voids carefully to avoid too much sanding later, as colloidal silica is very tough to sand.

Use the fiberglass discs to repair blisters that have penetrated the laminate.

Again make sure the voids have been cleaned with acetone and a clean rag. Mix up two to three pumps of resin and hardener. Saturate the area to be repaired with the resin, working it in to all the crevices. Using the smallest fiberglass disk first, place it into the repair and wet thoroughly with the epoxy. Continue using slightly larger disks until the void is filled. With the roller, carefully roll out the fiberglass so that it conforms to the shape of the repair and remove any air bubbles. Leave to cure and continue with remaining blisters.

Once the epoxy has hardened (about 24hrs) scrub the repairs to remove the wax-like residue then sand the filled repairs with 80 grit sandpaper to fair the surface or prepare the surface for more filling if needed. At this stage a mixture of colloidal silica and microballons can be used for the filler. This will make it easier to sand. Fill with the squeegee, allow to cure and sand again – this process is repeated until it is about an inch larger in diameter and circular.

All individual repairs should have a final coating of epoxy unless you intend to barrier coat the hull. Follow manufactures instructions for this and your antifouling paint.

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Rosie Burr
Rosie Burr
Rosie and her husband Sim Hoggarth on yacht Wandering Star have cruised the Caribbean and North America fulltime for nine years.

So Caribbean you can almost taste the rum...

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