Composite boat repair for the clueless

My actual introduction to working with composites came when I broke my composite Element. Well, actually that's not true. My real introduction to composites came when my parents used to build paddles and boats in a workshop out back when I was a young kid. Well, actually, I would get in trouble if I tried to go play in the workshop, since resin is toxic, and they didn't want me to start having liver issues before turning 10. So I suppose my real introduction to composites was when I built a composite deck plate with the advice of Jeremy Lauks. But that thing was a piece of junk.

Anyway, the point here is that I was paddling a composite Element on the Slave river in 2011, when there was a huge ton of wood coming down the river. I hit a couple stumps, which put a bunch of little cracks in the stern and a few other areas. However, the big one that finally made me get down to business was when I tried an airscrew on a shallow wave, landed upsidedown, and opened the bow up like an alligator mouth.

The wood situation that summer:

Slave River Flood 2011 - Knock on Wood from Leif Anderson on Vimeo.

The wave that did the deed, at a different water level, on a completely different day.
The damage was finally substantial enough that I stopped paddling the Element for a few days, and let it dry out enough to patch it.

So let's rap about composite repair for a bit. The basic deal is that there are a bunch of fibers (yeah, that's right. FibERs, not fibREs. I do what I want.) stuck in a bunch of glue. The fibers are your fiberglass or your carbon or kevlar or whatever. The glue is the epoxy. When the shit breaks, you need to glue replacement fibers on there. It's not really all that complicated.

Doing it Right

Now, if you want to do it right, you should sand away all the broken fibers, and replace them with fibers of the same type, oriented correctly to take the same amount of strain, and be sure that you don't have your patch overlap too much over the old fibers, since that would make the patch stiffer than the rest of the boat, which could lead to additional fractures at the edges of your patch. (This change in stiffness is known as a stress riser.)

But hey, if you were interested in doing it right, you wouldn't be here. You'd be talking to someone who knows what the hell they are doing. Like The Board Lady, or Easy Composites.

But you're here, so you're going to sit down and listen to my convoluted story of how one of my friends said one thing, then another one said something different, and I did this thing that was sort of halfway between them and it worked fine. FINE! As Nate Garcia famously said: "Then I just did it. And I was fine. When you're as swol as me, everything's easy." That's been my guiding philosophy for much of my tiny boatbuilding career.


So, I got advice from a bunch of more experienced people. Your usual basic patch would be to sand out a crack, wet out fabric with some epoxy, and lay it over the surface. When it cured, maybe there'd be a stress riser, maybe not, but it would be stiffer than it was with a freaking crack in it, and no water would be coming through. The problem here was that the crack was huge. I could stick stuff in there. There was no smooth surface to reinforce. This brought me to a dead stop.

The people I contacted were very wise, except for Peter Benedict, whose idea was way off base.

Celliers Krueger, founder of Fluid Kayaks and designer of the Element, told me a highly motivating story about breaking the bow off of one of his fiberglass boats and rebuilding it with a complex scheme involving chickenwire. If he could do that, I could fix this hole that was smaller than a finger.

The strategy that I ended up using was based most heavily on the advice of Kurt Braunlich, who is a slalom paddler and coach. When he finally saw the finished repair, though, he said that I had done such an ugly job of it that he didn't want me to tell people he was involved at all. Well guess what Kurt? Here you are.

As I mentioned earlier, the basic problem was that there was no surface to stick the stuff to. So we built one. First I spent a lot of time with a sander, getting all the useless broken stuff out of there. Then, on Kurt's advice, I made a chop glass mixture. This was super easy. I got some fiberglass fabric (you can order the stuff online, by the way), and I chopped it up. I ended up with a bunch of little short fibers, about 1/4 inch long. I mixed up a batch of epoxy and tossed in the chopped fibers. This made a gooey mess, which I slathered all over the place and smoothed out a bit.

Cured and sanded chop glass after filling the gaps

To make sure the chop glass cured with a smooth surface, I may have stretched a plastic bag over it. I honestly can't remember. The benefit of the plastic bag is that it gives a smooth surface for the epoxy to be squeezed up against, and it looks way better when you're done. For this repair, it didn't matter, because I had to sand this all down a bit once it cured anyway. Epoxy won't stick to the surface of unprepared epoxy. This is because of the blush - a really thin film that rises to the surface of a composite layup as the curing epoxy pushes out moisture inside the laminate. You almost always have to do some sanding before applying a new patch, and maybe also clean with rubbing alcohol.

So, once I had filled in this chop glass stuff, let it cure, and then sanded it down, it was time for the actual patch. The chop glass is great at filling spaces, but the short little fibers in there have almost no strength. It's fancy putty to make a base that a real patch can sit on. By the way, there are a ton of other space-filling putties, but this is the one that you can always make, since if you're about to patch your boat, you better have some epoxy and some fibers of some kind.

With all that prep done, the patch was stupidly easy. I cut out some fabric. I mixed some epoxy. I stuck the fabric on the boat. The end. The only subtlely is that it's hard to stretch the fabric so that it sits down nicely on really curvy surfaces like the bow. The trick there is to cut along the bias. Here is a nice short description of that. You basically cut along a line that's at 45 degrees to the angle of the actual fibers in the cloth. This magically makes the little strip you cut into this stretchy curvable thing that works amazingly well.

Well, in actual point of fact, I didn't know that back then. I just cut a big rectangle and slapped it on. This didn't lay down really well, and I didn't spread epoxy all the way to the edges. So it looked bad when I finished.
Really it's like an art form. It's the rebirth of cottage industry arts.
However, I sanded this off, and topped off the patch with another layer of kevlar, which doesn't tear as easily.
You have to look closely to even tell if there is a patch, right?

Reflections on this Episode

This was a big learning experience, and after this patch, I started feeling like I could patch anything. That was great. However, looking back from the dizzying height of two more years of patching stuff, I would have changed a few things.

First, that kevlar is very sensitive to UV degradation, and it always has these annoying edges, where the ends of the fibers don't lay flat, so there's this ragged edge. If you try to sand that edge down, you make it worse, since kevlar is freaking bulletproof, so it doesn't cut easily if you're just rubbing a gritty piece of paper against it. Instead it gets all frayed and looks terrible. So nowadays I usually have one more layer over the top of my kevlar. And what I should really have is an opaque layer over my kevlar. I'm working on a standardized procedure, but I don't have anything yet. Maybe an opaque layer of simple polyester fabric will be my eventual solution, or maybe epoxy pigments, or maybe I'll switch to using Innegra instead of kevlar. Who knows? The main point is, something should have gone over that last layer.

If you were keeping track, there were three completely separate preps and cures. This means the boat was out of action for a minimum of three days, since each cure is effectively overnight. Nowadays, I would probably have done this as one layup. I honestly don't know if this is better or worse than the multiple layups, but my suspicion is that it's ever so slightly worse, since there is the chance of an air bubble forming deep in this thick and complicated layup, which would affect the strength. However, this is a small chance, and we're talking about a whole extra day of paddling. Also, there are some benefits to the multiple layers at once:
- The multiple parts at once (chop glass, fiberglass, kevlar, and covering) can all exchange epoxy, so there is less chance of having undersaturated areas. Also the spongy mix can be massaged to squeeze out excess epoxy. Of course, if you are good at composites, you wouldn't have these problems, but this is a guide for people that have no idea what they're doing.
- We can be sure that the layers bond to each other, since there's no surface between them that needed to be sanded. Again, if you just prepare your surfaces carefully, this is not a problem, but I am terrible at making sure that I have sanded enough. I always get a little dust in the clean areas, so that they look sanded, then I clean off to get ready to lay up, and suddenly there are a bunch of little reflective unprepared spots on the surface I'm about to patch. That is not an issue if you're laying it all up together.

I would have also bagged the outer surface. Get some filmy plastic, like a trash bag (the orange ones are my favorites, but white works too. Black is too hard to see through.) Lay it on the wet patch, right after you take off your gloves. Then spend some time (like 5-10 minutes) rubbing the surface to massage all the air bubbles to the edges. It helps to lick your fingers. If you get all the bubbles out, you'll have a stronger and smoother patch, and it will look badass instead of looking like a scratchy ugly piece of crap stuck on the surface.

However, despite the things I would have changed, this patch is still going strong 3 years later. I hit it against a rock while getting cartwheeled out of control a few days later, which put a little extra crack on the side of it, but I patched that like it was nothing.


  1. Awesome Article..!!
    I never seen this type of article..really super and very useful information.
    Thanks a lot.
    MB Boats


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