Epoxy resins are increasingly being used to repair polyester/ fiberglass boats both above and below the waterline. The usage techniques are identical to those used in wooden boat building and described in Section VI of this book. The only real difference when using wood and epoxy is that wood is porous, at least for the first coat. Fairing and hole filling on a fiberglass hull is no different than doing the same thing on epoxy-coated wood. The same materials and tools are used.

The greatest use of System Three epoxy for fiberglass hull repair is gel coat blister repairs below the waterline. The product is used for many above waterline repairs as well. Polyester gel coats generally are not used as finishes on epoxy repairs. However, it will bond to System Three Resins’ SB‑112 epoxy system. We recommend the use of this product where the repair will be finished with polyester gel coat.

Above Waterline Repairs

The first thing before attempting to do any repair is to assess the problem. It is not possible to know how to fix something until you know why and how it broke. Professional repair yards understand this while many boat owners do not. Spend some time understanding the problem.

If, for example, a boat owner discovers that some fiberglassed in wood engine stringers are rotting; it will be necessary to pull the engine to affect a repair. Do this and then poke around to see the extent of damage. Don’t get out a grinder and start hacking away at the fiberglass in an effort to remove the cancer that affects your boat. In many cases fiberglass boat repairs using epoxy resin can utilize the existing structure to make a speedy repair. The very top of the fiberglass can be carefully removed and the rotten wood scraped out. A new piece the same size can be fitted, epoxy coated and glued in place using the fiberglass that was bonded onto the sides of the removed wood. The fiberglass top is then epoxy glued on the new wood and, presto! The new engine mounts are ready to go without a lot of realignment problems.

Think the problem through before mucking things up! Each problem and boat has its own peculiarities. Study the problem on your boat and use the principles of epoxy use described in this book. If you need more knowledge there are a number of books on fiberglass boat repair. You are probably not going to find that your boat fits any of the textbook examples in the repair books so you’ll have to make up your own recipe for success. It is doubtful that a can of some glop sold by some marine store will do the job. Study the problem, plot out the solution step-by-step, make a dry run in your head to see if you’ve missed anything, order the materials, get everything ready and go. This is the way the professional repair people do the new, unfamiliar repairs and you can too!

Gel Coat Blister Repairs

Much has been written about this increasingly common problem in polyester resin fiberglass boats. It is beyond the scope of The Epoxy Book to go into the “why” of the problem. System Three epoxy is highly rated by one consumer boat repair magazine for this purpose.

Many people and boatyards use our product in the repair of gel coat blisters. This section describes the method of repair. We do not claim that this is either the only method or the best method available. Our only claim is that the method described herein is being used and the track record has been generally good. There have, however, been a few failures using our products just as there have been using others’ products and methods. This section has not been written as an inducement to sell our product for this purpose. Blister repair, being an inexact science, is one where you “pays your money and takes your chances.” Please read the special warranty at the end of this section before deciding to use our products for gel coat blister repair.

We caution that gel coat blister repair is a dirty labor-intensive job. This is why the price the yard quoted may seem so high. Compared to the labor cost the materials are cheap. Unless you have more time on your hands than money in your pocket, you might want to accept the yard’s offer and have them do it. Or, you might have the yard do the gel coat removal and you do the rest with some occasional hired help. If you do plan to do the job yourself, make sure your haul out yard knows what you plan to do and allows it.

STEP 1: Clean Hull – Remove Bottom Paint

Remove all marine growth, scum, barnacles, etc. Your yard may do this upon haul out by hydro blasting or steam cleaning. It may be necessary to use a scraper to get the barnacles off.

If your boat lacks a boot top stripe you’ll want to develop a technique for marking the top of the bottom paint line. Running masking tape above the line on the topsides is a good method. It will become frayed when sanding and you’ll want to replace it for Step7. Making small grease pencil marks right above the tape on the topsides every foot or so will serve as a guide for the new tape. These are easily removed with soap and water or paint thinner when the project is finished.

When the hull is dry you’ve got to make a decision whether you are going to remove the gel coat or merely abrade it by sanding and opening the blisters as discussed below. The decision will largely hinge on the extent of blistering. Removal offers the greatest chance of a complete cure but it also requires great labor to bring the hull back to its original fair condition. Merely sanding but not removing gel coat eliminates a lot of the fairing problems but may miss some of the small blisters. They may show up on next haulout and you’ll have to patch them then.

Sandblasting is the easiest, fastest and most widely available way to remove the gel coat. Several newer methods that work like a powerplane or joiner have been developed but the equipment is expensive and not yet widely available. It is worth paying a professional to remove gel coat. Your yard may know of someone who does this. Be sure to check with your yard to see if they even allow sandblasting. Some do and some don’t. If sandblasting, be careful to remove only the gel coat and any damaged mat. Digging into the hull with the sandblaster will weaken it as it removes structural fabric.

A 1500 to 2500 rpm sander polisher with an eight-inch foam backed pad is the best way to sand gel coat. Be advised that this is dirty, strenuous and tedious work. You can do it yourself but will get very tired and may spend as much money in time and materials as you’d have paid to have someone come in and sand it for you. Hulls with gel coat removed dry faster than those with the gel coat intact.

Bottom paints contain toxic materials. Avoid breathing dust or getting dust in cuts or open sores. Always wear suitable dust masks. Wash contaminated clothing separately from other clothing.

STEP 2: Open Blisters -Remove Damaged Fiberglass

If you have elected to remove the gel coat you have already completed this task. Skip on to Step 3. If you have only sanded the gel coat and do not plan to remove it, read on.

Now is the time to open the blisters and clean them out. Use the point of a utility knife to puncture each blister. Insert the knife and with a twisting action, cut out the damaged gel coat and fiberglass. Remove all the “rotten” material. Keep cutting until you get it all out. Don’t worry about cutting good fiberglass. It is highly resistant to cutting. Use the knife to get rid of all undercuts, as they will make filling more difficult.

Other tools may be used also. Small rotary files attached to electric drills have been successfully used. The idea, whatever you use, is to open up the blisters and remove damaged gel coat and fiberglass. Blisters generally contain acidic water under pressure. The water may contain dissolved material, which could cause eye irritation or damage. Wear safety goggles and stand back out of the line of fire.

STEP 3: Wash and Dry Boat and Blisters

Wash the boat thoroughly from the boot top stripe down with fresh water to remove all traces of salt, blister fluid, sanding dust and other dissolved material. Rinse the hull well. Be sure to squirt the water into the exposed blisters to remove any contaminants in the blister. Let the boat drain and air dry for several hours. Look and see if any purple-brown colored vinegar smelling liquid is oozing out of opened blisters. This is blister fluid. If it is, then dig out those blisters even more and rewash. Repeat this step as necessary.

The next step, drying the hull, is the single most critical operation to affect a cure that lasts. It is of paramount importance that the hull be as dry as possible. Start by emptying the bilge of standing water. In 80°F weather at 40 percent relative humidity the average blistered hull will take three weeks to dry to a steady low level. You may not be able to achieve these conditions without “skirting” the boat and using heat and a dehumidifier. If you plan to do it this way we recommend reading “A Manual for the Repair of Fiberglass Boats Suffering from Osmotic Blistering” by Richard and Roger McLean.

Some people have suggested that the hull drying process can be accomplished by vacuum bagging. We have studied the results of this process and talked with those who have done it. While there is some initial drop in hull moisture content, this method will not properly dry a hull even at safe elevated temperatures. We cannot recommend this method of hull drying. If you live in an area where boats are hauled in the winter do Steps1, 2, and 3 in the fall and when the weather starts to warm in the spring, skirt the boat and finish the drying.

STEP 4: Fill the Blisters

When the hull laminate has completely dried you should roll on a sealer coat of mixed Clear Coat epoxy resin/hardener. Work the mixed resin into each cavity to wet out any damaged fiberglass. Allow it to soak in for an hour or two. Then mix up some System Three epoxy and make a tilling putty by the addition of microballoons and silica thickener. This material makes a non-sagging putty which will replace the material you removed in Step 2. Try to perform this step on the shady side of the hull if possible as you will have longer working time.

Initially, mix small batches until you get the hang of working with QuikFair or an epoxy/microballoon mixture. You can always mix more but once mixed you’ve got to use it within a short period of time or it will go off in the pot. Fill each blister from the bottom (otherwise you will trap air) using a putty knife or similar tool. Fill flush with the gel coat surface with a slight overfill which will be sanded down later. Finally, use the edge of the putty knife to scrape off any excess around the perimeter of the hole. Get it now before it cures or you will have to sand it off later.

Fill all the blisters and allow to cure at least overnight if the temperature is above 60°F or two nights if the temperature is below60°F before proceeding to step 5.

If you have had the hull sandblasted then you may not have blister pockets to fill. Your job is to begin fairing. Before beginning, roll on a coat of mixed Clear Coat resin to seal the exposed fiberglass surface. Allow several hours to cure before fairing. The idea in fairing is to restore the surface to the gel coat level prior to removal. You will do this with the same with QuikFair or the microballoon mixture but use a broad knife or similar tool to apply it. In effect you will be plastering the hull with the epoxy microballoon mixture and sanding it to get it fair. A careful job applying the “mud” will save hours of sanding later.

STEP 5: Sand the Hull

Use 60 grit aluminum oxide paper and sand the filled cavities fair with the surrounding hull. Blocks or sanding pads help avoid sanding the cured putty below the surrounding hull surface. The putty will sand faster than the fiberglass. Refill any concave holes or exposed air bubbles with the putty blend. Allow to cure and resand.

If you removed the gel coat and puttied the entire hull bottom you will now sand it fair. This is best done by two people using a long board. This is just a long sanding block with paper glued to it. The flat part of a straight 2x4 about 3 feet long works well. You may find that the sanding will reveal low spots that require additional microballoon mix. Fill them, resand and continue in this fashion until the entire hull is without ridges, bumps or hollows.

STEP 6: Prepare the Hull for Epoxy Coating

After the cavities have all been filled and the hull is fair it is necessary to prepare the hull for epoxy coating. It is this coating that will help prevent the hull from blistering in the future as the epoxy coating is much more resistant to water penetration than the polyester resins used to build your hull.

Begin by sanding the entire hull to be epoxy coated with 60-grit paper if you have not sanded it in the filling/fairing process. You may hand sand it or use a vibrating sander. Rotary high-speed sanders should only be used if you are confident about your ability to use them. They are heavy and cut fast and you may end up gouging the hull. Sand the hull until there is no gloss left - sand right up to the old bottom paint line. Avoid breathing the dust.

After the hull has been thoroughly sanded wash it with water to remove the sanding dust. Really get in there and scrub it with a clean brush to remove all traces of sanding dust. Rinse and allow it to dry well - at least overnight.

STEP 7: Coating the Hull with Epoxy

You will need System Three epoxy resin and hardener, disposable gloves, graduated cups, stir sticks, yellow foam roller covers, roller frame, disposable brushes and a 9” roller paint tray for this step. Use only the roller covers supplied by us. They are designed for our product.

The idea here is to get on a minimum of four coats of epoxy without the runs and sags that will require a lot of sanding later. It is this coating that provides the barrier that helps prevent the future intrusion of water into the hull. First run masking tape around the boat so that the bottom edge of the tape is right at the top of the old bottom paint line. This will help prevent rolling epoxy on the topsides or boot top stripe.

Put on the gloves and mix up about six ounces of resin/hardener. Pour the thoroughly mixed material into the roller pan and “paint” the hull using the yellow foam roller covers. Put on as thick a coating as possible but not so much that it will run and sag. Experience will teach you how much you can get away with. Better to spend the time putting on an extra coat if the previous coats have been a little thin rather than sanding out runs later. If you see a run developing go back and roll it out. The resin/hardener mix contains no solvent so you won’t leave marks if you do this. Brush out any air bubbles with a foam brush using light strokes. Just use enough pressure to break the bubbles and not disturb the uncured epoxy.

Remove the masking tape right after you finish the first coat. Do it before it cures or else it will be difficult to remove later. Wear gloves since the masking tape will be wet with epoxy.

Retape then apply the next coat as soon as the previous coat is set enough so the combination of the two coats will not run and sag. This is about 2 to 3 hours with fast hardener on a 60°F day. Less on a warmer day, longer with slower hardener. You may wait up to72 hours between coats without sanding as long as the hull does not become contaminated in the meantime.

Put the succeeding coats on the hull right up to the top of the first coat. By catching the light right you’ll be able to see where the first coat ended and the new coat starts.

If you wait longer than 72 hours between coats you’ll have to give the epoxy a light sanding to give the new coat some “tooth” to tie into the previous coat. Prior to this time the coats will bond chemically to each other.

Allow the last coat to cure overnight before proceeding to the next step - even longer if the temperature is below 60°F during the cure cycle.

STEP 8: Sanding for Bottom Paint

Hose and sponge the hull with water to remove any oily surface film on the cured epoxy. This is a water-soluble film and will be thicker if you applied the coating in humid weather. It is a by-product of the epoxy curing reaction. Solvents do not remove this film.

Sand the cured epoxy with 80-grit paper to smooth any runs and kill the gloss. A sanding block helps to prevent over sanding when removing cured runs. Be careful not to sand through the epoxy coating and re-expose the gel coat thus losing the epoxy protection. If you desire an even smoother bottom you may want to use finer grades of wet and dry paper. If the sandpaper clogs excessively then the epoxy coating has not sufficiently cured. Wait another day. Wet sanding helps prevent paper clogging and keeps the dust down. Use wet or dry sandpaper and dip it into a bucket of water occasionally.

After sanding, wash the dust off with a sponge and water. Allow the hull to dry. Wipe with acetone, MEK, or the solvent recommended by the bottom paint maker for preparation to applying this paint.

Do not be seduced into believing that bottom paint can be applied to partially cured System Three epoxy. Some would have you believe that this avoids final sanding of the last epoxy coating. If you do this you may find that the bottom paint along with the last coat of epoxy will start to fall off in several months.  

STEP 9: Applying the Bottom Paint

Follow the manufacturer’s recommendation when applying bottom paint to the epoxy-coated hull. Most bottom paints will adhere well to the sanded epoxy coated hull with no primer. Keep in mind, however, that bottom paints are formulated for polyester gel coat hulls and some may not work well on epoxy coatings. You should do a small test patch on your hull to make sure that the bottom paint dries properly and adheres well to the epoxy coating.

Estimating Amounts

Figure 0.2 gallons of mixed resin and hardener per coat per 100 square feet of hull surface (500 sq. ft. per gallon per coat) to be coated for the barrier coat. Thus, a 30-foot sailboat with 175 square feet of under water surface will require about 1 3/4 gallon of mixed resin/hardener for the barrier coat. The amount necessary for the filling/fairing will depend greatly upon each boat.

Materials List

The following materials are available from System Three Resins and will be necessary for the completion of a blister repair job:

Epoxy Resin and Hardener, Filler Materials, Measuring Cups, Mixing Sticks, Mixing Pots, Disposable Gloves, Disposable Brushes, Foam Brushes, Roller Covers, Roller Frames, Roller Tray, Squeegees, Masking Tape, Dust Masks, Utility Knife, Sandpaper, Safety Goggles

In addition to these materials you will need hoses, buckets scrub brushes, clean rags, paper towels, putty knives, a ladder, sanding tools, etc.


Because the construction of your hull and the repair of the gel coat blisters are beyond the control of System Three Resins, no representations or warranties are made or implied that future gel coat blistering will be prevented using the techniques and materials described in this booklet. System Three Resins shall not be responsible for incidental or consequential damage as a result of using its materials or the techniques described herein. System Three Resins’ sole liability shall be the replacement of defective materials or the refund of the purchase price of these materials.

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