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The Epoxy Cure

by Jim Hebert,

The Problem

The hull of my recently acquired 21-year-old Boston Whaler had a few small nicks, gouges, and scrapes that needed attention. Most of these were near the bow along the keel and spray rails, probably caused by little collisions with the boat trailer in the process of launching or recovering. Instead of taking the boat to a fiberglass repair shop, I decided to fix these blemishes myself. They were not just a cosmetic problem, because any breech in the watertight integrity of the Boston Whaler hull could allow the foam-filled interior to become saturated with water.

Becoming An Expert

Since the company that built my boat is still in business, it seemed natural to call them for advice. I gave Boston Whaler Customer Service a ring at 904-428-0057. Dennis, a friendly Customer Care Representative, described the repair process. First, he explained the Whaler hull construction (more on that below) and how I could assess the damage. To repair it, I should get some polyester resin, mix in some chopped fiberglass, and finish off with gelcoat. A third-party firm, Spectrum Colors, could supply me with the correct color gelcoat, and the rest of the materials were available at the boat store. Great--it sounded do-able, even for a new-to-resins guy like me.

The first step was to contact Spectrum Colors, in Auburn Washington at 253-735-1830. I gave them my boat info, the hull serial number and some other numbers stenciled on the transom. I described the hull color as "off-white". They were confident they had the original color gelcoat, and I ordered one of their 2-oz. Gelcoat Patch Paste Repair. This costs about $19.25, delivered via Priority Mail.

Next, I went to a marine store, where I found they had a good inventory of repair products, along with plenty of literature on How-To-Do-It. There seemed to be three categories of fiberglass materials: little tubes of pre-tinted gelcoat for filling minor scratches; pints, quarts, and gallons of resins for building whole boats; and some "repair kits" for restoring minor damage. Among these kits, epoxy resins seemed to dominate, so I settled on a epoxy product from WEST SYSTEM® . I'd heard of them before, and since they're a Michigan company, they were the hometown favorite.

Having never done much else with epoxy than use it as a glue to fix a few broken household items, I needed some background on it for marine use. I bought the WEST SYSTEM® publication,

Fiberglass Boat Repair & Maintenance--A guide to repair, restore and prolong the life of fiberglass boats with WEST SYSTEM® Brand Epoxy.

This booklet became my bible, and what I am going to tell you here is really a narrative of my experiences trying to implement their advice. I doubt that I have invented anything new, but you may find my experiences useful.

I left the store with a bundle of stuff, including:

JUNE, 2000 UPDATE: What I didn't know then is that epoxy is a much better choice for these repairs than polyester resin. The bond formed between the laminate layers in initial construction is known as a primary bond. The materials are freshly laid together and are still curing. When making repairs, you will be creating a secondary bond, attempting to adhere materials that are already completely cured. Epoxy is a much stronger adhesive than polyester resin, and epoxy will produce a stronger secondary bond. In addition, epoxy does not contain styrenes which are present in polyester resins. Styrenes will dissolve the foam core of the Boston Whaler. For more information on epoxy repairs, listen to this interesting interview.

Read First, Repair Later

I read through the literature several times, deciding that Section 2, "Repairing & Finishing Minor Cracks & Holes" was most appropriate for my situation. Once I understood the instructions, I began the repair process. Like painting, the secret is in the preparation. I changed into some old clothes, and moved out into the garage to work on the boat.

Day One: Sanding and Prep

Before sanding, I washed the area and wiped it down with Acetone to remove any dirt, wax, and grease. You don't want these foreign materials to be sanded into your repair. Also, prepare yourself. You need to dress appropriately, because this is not going to be like waxing your boat in a bathing suit. Wearing a filter mask, eye goggles, shower cap, long-sleeves and gloves, I began to (dry) sand the gouge along the spray rail with 100-grit paper. Of course, the dog chose this moment to launch into a barking frenzy at the new neighbors next door. I had to go retrieve her, dressed like this. I think the new neighbors are a little skeptical about me, now, after seeing me in a pink shower cap. Scratch one new neighbor; back to the boat.

The dust was really flying. Take precautions against getting this into your eyes, lungs, or even on your skin or hair. You will regret it if you don't. I was quite astonished with how fast one can remove fiberglass material with relatively fine sandpaper like this. It gave me something to think about the next time someone suggests I beach this boat on a nice stretch of sand. Abrasion resistance is not fiberglass's long suit. In a few moments, after I removed all the loose and damaged material and got down to solid boat again,the size of the injury (to the boat) had doubled.

The typical fiberglass boat laminate consists of an outer, pigmented, solid resin layer ("the gelcoat"), followed by underlying layers of resin saturated cloth ("the laminate"), and (in the Boston Whaler case) foam. On my boat the gelcoat layer is off-white, the cloth laminate is a bluish color, and the interior foam is whitish brown. This makes it easy to see the depth and extent of an injury. In my case, the gouges were down through the gelcoat and had exposed the top layer of the blue laminate.

Once I had the area sanded, I washed off the dust with soap and water, dried it with clean paper towels, then wiped it down with Acetone again. This made the perfect surface for epoxy to adhere to: clean, dry, and plenty of surface texture.

Mixing Epoxy

One great thing about the WEST SYSTEM® repair kit: it provides you with pre-measured amounts of resin and hardener. Ever since my first attempt with a household epoxy adhesive 30 years ago--which probably has still not hardened-- I have been a little nervous about mixing epoxies. With the "catsup packages" of ingredients, your first batch of epoxy is pretty much guaranteed to cure properly.

I squeezed out the resin and hardener, and mixed them together in the supplied mixing bowl, using the supplied mixing stick. You really get all the things you need in this "kit." Once you mix hardener and resin, you have about ten minutes of "pot life", after which the epoxy begins to thicken and becomes difficult to apply.

Using the supplied small brush, I brushed the epoxy mixture into the damaged spray rail. The first coat of epoxy should be unthickened, so it can easily wet the hull laminates and form a good bond. I soon discovered that I had plenty of epoxy left over after this initial "wet-out" of the repair, so I decided to jump to step two immediately.

I added some WEST SYSTEM® 404 filler. This is a white powder they describe as an all-purpose high-density filler, suitable for gap filling like I was doing here. Where did I get this? It came in the kit, of course! To the remaining epoxy mixture I added enough 404 to thicken the resulting compound to the consistency of mayonnaise. I troweled it into the gouge on the boat, and sculpted it to fill the gap as best I could.

Now comes the hard part, so to speak. You have to wait 24 hours for the resin to cure and harden. "See you tomorrow," I said to the boat, and went into the house to shower up. I do have to confess, however, that I checked on the progress of the cure several times that afternoon, trying to make sure it was going to harden. The psychological scars of that 30-year-old uncured batch still remained.

Day Two: Expanding Your Work

Each of the small "kits" has enough material for two batches. Now that I was acquainted with how much epoxy I would have from a single "catsup package" batch, I decided I'd better prep more areas on the boat for repair. Otherwise, I would be wasting a lot of epoxy. Accordingly, I turned my attention to the scrapes along the keel. Soon I had opened up a two-foot-long wound along the leading edge, once I removed all the loose material. Funny how these projects grow, isn't it?

Comparing the hull color to the new epoxy-plus-filler color, it was apparent that some tinting was in order. It wasn't clear from my reading exactly what sort of pigments to use, so I called Bay City, Michigan and talked to the guys at Gougeon Brothers, the makers of WEST SYSTEM®, to get their first-hand advice. I learned that I could use acrylic pigments available at an artist-supply store to tint the resin. This was good news, because those pigments are cheap, available, and come in a zillion colors. I took a look at the hull color with an artist's eye, and drove off to get a $4 tube of acrylic paint. To make my shade of off-white, it looked like some Raw Sienna would be the proper tint. I also needed white pigment, too, as the raw epoxy color was really more like a yellowish varnish finish.

Day Three: Tint to Match

The recommended amount of tint that can be added to the resin is 5%. The cured epoxy that results is not completely opaque, and so it does not entirely hide the underlying color in a single coat. My initial repair to the spray rail was already pretty well built up, but because I had not tinted the epoxy, you could see a bluish cast to the repair, coming from the underlying laminate layers. When I mixed up a third batch of resin, hardener, filler, and pigments, I was able to arrive at an excellent color match. Applying it hid some of the earlier repair, but some of that bluish tint showed through.

By this point, the gouge had been filled completely, and when sanded down the repair was very well faired into the spray rail. Except for the mis-match in color (from the bluish tint), it was pretty well invisible. For a first attempt, I was very pleased. And since this part of the hull was pretty much out-of-sight once the boat was in the water, it did not really matter that there was a slight color mismatch.

Day Four: Finish Layer

When the epoxy layers were completely cured and hardened, I decided to experiment with gelcoat. By this time, the "factory color" gelcoat had arrived from Spectrum Colors. There must have been some misunderstanding about the hull color, because the small 2 oz. jar they sent was not close to being the correct shade. The folks there were very nice; they agreed to let me send it back, and they would try again.

Martha Stewart To The Rescue

To more effectively communicate the hull color to Spectrum Colors, I came up with an idea: get some paint chips, find the one that matched the hull, and send a little sample of the color to the Gelcoat maker. This idea came to me while shopping at K-Mart, so I grabbed a couple dozen different shades of "off-white" from their new line of Martha Stewart paint. I must have had 48 different shades of white, but I discovered that none of them even came close to matching the hull color. For the next couple of days, every time I passed a paint store I stopped in and grabbed paint chip charts of off-whites. Finally I found a shade ("bone white") that came close. I sent it off to Spectrum Colors and crossed my fingers.

JUNE, 2000 UPDATE: The Spectrum Colors line of OEM matched gelcoat repair kits is now being handled by West Marine. You can probably find it in your local West Marine store. Also, to obtain pigment to match a gelcoat color, you could also attempt to purchase just the tint material from the paint store. First, find a paint chip chart with the matching color. Then ask the paint store just to sell you the various tint pigments in the proper ratio. Combining these together and mixing them with white-based gelcoat should create a close match. I haven't tried this myself.


The makers of epoxy have plenty of wonderful things to say about it. It is strong. It adheres well. It is waterproof. The one thing they can't say, however, is that it resists Ultra-Violet light. Therefore they recommend covering it with a final layer of something else, like paint or gelcoat. Otherwise, the epoxy is likely to yellow from the UV in sunlight.

More reading about gelcoat got me up-to-speed on it. Down to the marine store I went, this time coming away with:

I also stopped at an automotive paint supply store, where I bought an interesting device called a "Preval Spray Gun" ($4). It is a disposable portable spray gun, into which you can load your own paint. It has enough propellant to spray up to 16 oz. of whatever you load into its little glass spray jar. [NOTE: This, too, is now available from West Marine.]

After some more surface preparation, I got ready for my first attempt at gelcoating. When I opened the quart can of gelcoat, I was shocked, twice. First, the consistency of the material was like peanut butter. How would I every get this through the sprayer? Second, the smell was intense. It's that "new boat" smell you experience at the boat show when you board a boat that has just been built. Only now, it was the "new boat" smell multiplied by one hundred. Compared to epoxy, this gelcoat stuff stinks.

I troweled some white gelcoat out of the can and into a mixing bowl. I coaxed a little dab of brown tint out of the tube and into the gelcoat. Wow! This tint was powerful. Instead of a beige or tan, I had medium brown gelcoat. I grabbed another mixing bowl, added some fresh white gelcoat to it, and began mixing again, this time using the first batch as the tint material. Slowly I approached the shade of off-white I was aiming for. Into this I added some styrene to thin down the mixture, but only about 10% by volume, the recommended maximum. This did thin the gelcoat, but it still looked too thick to me to spray. The smell was so bad that I could not really imagine making this material airborn, so I decided to brush it on.

JUNE 2000, UPDATE: I think the Pro's add acetone to dilute the gelcoat enough to spray. The acetone quickly flashes off the applied surface, leaving the gelcoat behind. Again, I haven't tried this myself yet.

I still had not added the hardener to the mix. The hardener is supplied in the proper proportion for the whole quart can of gelcoat, about a half an ounce in a little tube. It says that if you use less than a full can of gelcoat, you should add the proportional amount of hardener. This gave me a scare. I'd only used a couple tablespoons of resin. That meant I needed just a few drops of hardener. I gave it my best guess, adding a drop or two extra to be sure. This mixing was not as simple as the pre-measured epoxy had been.

I brushed the gelcoat on, trying to get as thick a coat as I could. I figured I could sand out any brush marks. When I had done my best, I then turned to the PVA.

One interesting thing about gelcoat is that it will not cure if in contact with air. In building boats with female molds, this is not a problem. The gelcoat is the first layer sprayed on the mold. One side of that gelcoat layer is in contact with the mold, the other side gets covered by more resin and laminates. So it cures hard because it is not exposed to air. Applying gelcoat to the exterior of the hull, as I was, leaves it exposed to air, where it will only cure to a tacky finish. That is when the PVA (Poly-Vinyl-Alcohol) comes in. In boat building, I think they use this on the mold surface as a release agent. In my case, I used my Preval sprayer to spray it over the gelcoat. This allowed the gelcoat to avoid contact with air. After a few hours, I washed off the PVA with soap and water. The gelcoat underneath was cured to a hard finish. I guess I had added enough hardener.

JUNE, 2000 UPDATE: I wouldn't use the Preval sprayer for the PVA again. Instead, just use one of those common spray handle bottles (like those many cleaner products are sold in) to spray it on. Be sure you wash the bottle of any previous residue before using it for the PVA. Another method of excluding air is to overlay a sheet of plastic wrap material. Apparently this won't bond to the gelcoat, which will then cure to a hard finish underneath it, and the plastic wrap can be peeled off. Again, I haven't tried this myself.

Day Five: Sand, Rub, Wax

I left the gelcoat to cure for a week or so, to make sure it was fully hardened. The next Saturday, I sanded the gelcoat. By this point I had modified my approach to sanding. I now always wet sand. This drastically reduces dust, and it prolongs the life of the sandpaper, too. With a sponge and a bucket of clean water, you can wipe the area clean and get a good impression of how you're doing. Rinse the sponge in a second, dirty water, bucket. Wear the same protective clothing, although perhaps you don't need the the mask. And watch out for rivulets of water coming off the boat and running down your arms. You'll be itching later when the water slurry dries and leaves powdered boat hull on your arms.

Starting with about 200-grit, I wet-sanded the repaired areas, working my way up to 600-grit paper. After that, I switched to rubbing compound. Finally, I washed and waxed the whole region of the boat I had been working on. The final results were pretty good. The color match turned out to be a little off. The literature indicated that you should allow for some darkening of the gelcoat as it cured, but this did not seem to be realized in my case. I had left the tint a little on the lighter side of the target color, and as a result, the repaired area is now a little lighter in color than the rest of the hull.

I am rather pleased with the results, even allowing for the color mis-match. The repaired areas are very well faired into the hull, and they look strong and watertight. Now that I am familiar with using epoxy and gelcoat, I would be willing to take on more repairs in the future. I might do a couple of things differently. For one, I'd begin tinting the epoxy to match the hull right away. Adding a little pigment does not thicken the epoxy much, so it should not reduce the wetting of the crucial first layer. Second, I'd save room in the build up of the repair for a couple of final layers of tinted epoxy without fillers. This should make for a better water barrier. Finally, I might dispense with the gelcoat layer altogether and just paint the topcoat on, finishing with a couple of coats of wax. Considering how awful the gelcoat is to work with, I am not sure it is worth the bother. It's only real purpose in my case was to provide a strongly pigmented, opaque, and UV-resistant layer. The epoxy underneath was supplying the strength, fill, and water resistance. Simple paint could replace the complicated gelcoat.

I did get another batch of gelcoat from Spectrum Colors a few days ago. This looks more promising for color matching, and I plan to use it to fill some minor surface scratches. However, the weather is now against me, as cooler fall temperatures are not good for curing resins. And in fact, as I am writing this, we are getting our first snowfall of the season here in southern Michigan. That is always a sign that boating season is over. Now it's "wait till next year" for me and further repairs. I feel much more confident about being able to produce professional results when I tackle filling a few extraneous holes in the deck next spring.

DISCLAIMER: This information is believed to be accurate but there is no guarantee. We do our best

Copyright © 2002, 2016 by James W. Hebert. Unauthorized reproduction prohibited.

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