Not sure what boat hull blisters are? If you own a boat you will probably be familiar with them. Blisters are usually just a cosmetic problem but they can be much worse if they reach the fiberglass laminate below.  They occur on the hull of the boat. There may be just a few or there could be hundreds of them.  A blister is a de-bond of paint or other coating from the part surface. It is an undesirable rounded elevation of the surface of a plastic with boundaries that are more or less sharply defined. It resembles a blister on the human skin.
Blisters occur through osmosis. Polyester resin and gel coat are fairly water permeable but water can still pass through. If there are small voids, the water will get into them. The water has a chemical reaction with the resin used for making the hull.  The chemical reaction creates an acidic liquid that needs more water to reduce the acidity. This causes it to expand, thus creating a blister.

It is important not to panic if you see blisters on your hull. You will want to first check how severe the problem is. The ideal time to check is immediately after the hull is removed from the water. Shortly after removing the boat, the blisters will deflate and you will not be able to see the extent of the problem. If you are not around when the boat is removed from the water, you can lightly sand the paint with a coarse sandpaper. The sandpaper should get the raised part of the blister so you can identify where they are.

First, break open the blisters with a utility knife, chisel, drill etc. Wear safety glasses and safety clothing when doing this. The liquid inside of the blister may come out forcefully and you do not want to get it in your eyes. Once the blister is open, tap around the edges to make sure you have completely opened the damaged area. Wash the area thoroughly to get rid of all contaminants. You could use rubbing alcohol (isopropyl) and clean paper towels. Use a drummel or grinding bit to grind away the rotten, decomposed material. Make sure to get it all out of there. Keep cleaning out the area until you reach the good gel coat layer. If the laminate looks damaged you will need to get rid of layers of that also until you reach the healthy fiberglass material. The laminate is good when you do not see any white fibers. It will look dark and transparent. When the bad material is ground out you will have what looks like a dimple in the hull.

You can use a garden hose to wash the hull but a power washer is the best option.  It will help you get rid of any leftover acidic liquid, rotten fiberglass and other contaminants. After washing it well, it will need to dry out for several days to a week.  The power washing process should be repeated again, letting it dry out for another week afterwards.  Making sure the hull is completely dry is a MUST.

There a couple of ways you can check to see if the boat hull is completely dry.  One way is to use an expensive moisture meter. The inexpensive route would be taping some plastic to the area.  If you choose the plastic option, leave the piece on for awhile.  If the hull is still drying out, there will be condensation that comes out onto the plastic.  If this is the case, take the plastic off, dry the area and re-tape. Repeat until there is no more condensation on the plastic. To speed up the drying process, you could tent the bottom half of the hull, taping some plastic at the water line, and place fans underneath the plastic.  Make sure anything wet, such as sails, are removed from the boat.

When you are positive that the area is 100% dry you can start the repair.  Epoxy resin is the preferred resin for this type of repair but vinyl ester resin is another good option.  Start by mixing a small batch of resin and apply a layer on the repair area with a brush. This will seal the area. Once the epoxy is partially cured, stiff but still tacky, you can replace the fiberglass that was ground away.  A mid size cloth such as a 6oz, 7.5 oz. or 10 oz. weight would work well. (If the fiberglass laminate was not damaged you can skip this step).  The fiberglass should be cut into round pieces that are just a little bigger than the depressed area. Add epoxy resin to the cloth. Test out one or two blisters to practice.  You want to know what you are doing before repairing all of the blisters. This goes for when you are adding epoxy putty (which comes later). It is definitely best to practice. Working with fiberglass and resin is an art not a science.

Mix a batch of epoxy resin with a filler such as collodial silica, also known by the brand names of Cabosil or Aerosil. Mix in enough filler to become a creamy peanut butter consistency. Use a spreader or putty knife to apply the mixture.  Make sure you do not over fill the depressions.  You will want it flush or slightly below flush because the epoxy/cabosil mixture is very hard to sand. The more sanding you have to do, the more time consuming the job will be. Also, you do not need to worry about epoxy resin shrinking. Let it cure overnight and then sand the repair area until it is flush with the surrounding area. Finish with another coat of epoxy resin (no filler added) over the top. You can then add whatever bottom paint you choose.

Unfortunately more blisters can arise later but hopefully this work will solve the problem.

*PRACTICE, PRACTICE, PRACTICE!  These are general guidelines only.  Not every repair is the same. See what works best for your particular project. *

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