Many builders have asked for my advice on epoxy resins, and I would like to share my personal experiences. But first, I need to preface my remarks by emphasizing that there are many epoxy resins on the market today, and I am sure many of them are excellent for building stripper canoes and kayaks. Unfortunately, I have used only a few brands, and so those are the only ones I am able to comment on from my own experience. If you want to use one of the other products and you want advice or feedback from previous users, you'll have to try the boatbuilder's newsgroups or some other means to find out more about them.
I have used four different resins from three manufacturers: West Systems 105 Resin with 206 Hardener, West Systems 105 Resin with 207 Hardener, System Three Clearcoat, and MAS Low Viscosity Epoxy Resin with Slow Epoxy Resin Hardener.
West System makes a fine range of products and is widely distributed. The 206 Hardener listed above is usable for strip building, but is not recommended, mainly because it is not all that clear a finish. If you were planning to paint the hull, it wouldn't matter, however. The 105 Resin with 207 Hardener is intended for wood strip hulls, and cures very clear, however with a yellow tint, but this added colour isn't at all objectionable over wood in most cases.
Ten years ago, when West System was the most common resin for strip building, it became a mainstay among canoe builders. However, the relatively fast cure meant that you had to work quickly and know what you were doing. It was necessary to master these techniques, and I for one found it was difficult to use well. Because it is a fairly high viscosity resin, it was slow in soaking into the fiberglass cloth, so you had to apply it fairly heavily. But when it had soaked in for 15 to 20 minutes or so, you had to squeegee off the excess resin. If you squeegeed a few minutes too soon, you could squeeze out too much resin and starve the cloth. Leave it too long, and you couldn't get enough resin out of the cloth, and in some spots it might have too much resin under it creating lumps and bumps and runs. In my case, invariably I found that just when it was the right time to be squeegeeing the first batch I had applied, that was the time I was supposed to be applying the third or fourth batch as I worked my way down the hull. Sometimes, runs would develop in a section I thought had been properly squeegeed. I know many a pro used this stuff with nary a problem, but I never got enough practice to get good, and I dreaded this stage.
A few things that I found helped a bit with using West System epoxies included temperature control and having a helper. With a helper, one person could just keep on applying resin, while the other person did the squeegeeing. Then, you just had to get the timing right... The other thing was temperature control of the shop. If you were doing this when it was cool out, you could gain additional working time by cooling the shop to 60 degrees F or thereabouts, and if you work fast, you could get all the resin applied before you have to start squeegeeing. Then, warm the room to speed up the cure.
Today, I still use West System resins, but I tend to use them mostly for fill coats, to fill in the weave of the cloth after the wet out coat. In this use, the higher viscosity and faster cure help you get a suitable build up in a shorter time. For these reasons, it is also excellent to use with the appropriate West System fillers as a glue or fairing compound. For example, I use it frequently with the Type 407 Low-Density Fairing Filler to make the glue that bonds in my black walnut decks and inlays, because the brown colour is virtually indistinguishable from the walnut. If you accidentally got a bit of a low spot in the finished hull (maybe you oversanded a spot), it is even possible to build up the resin with a few extra coats in this area only, and then sand it all smooth again so that the low spot will never show!
A few years ago, John Winters introduced me to System Three Clearcoat. This is a very low viscosity slow cure resin, and it's very good for wetting out the cloth on the first coat. It wets the cloth quickly, and soaks in so that you do not need to squeegee (hooray!) The very slow cure means that you can take your time, wet out the entire hull, and still be able to fix problem areas even hours afterward.
There are two things you need to watch out for when using System Three Clearcoat: first, because it is such a low viscosity, some of it will soak into the wood underneath and may starve the cloth. If this happens, it may not become apparent until after the boat is done and it gets out into the sun. It can develop hundreds of tiny blisters as the little bits of air trapped between the fibers of the cloth expand. There is no cure to fix this once it has happened. To avoid this, you must either apply a coat to the bare wood before you apply the fiberglass, or, make sure that you apply the resin quite liberally, using as much as you can without it producing runs, to make sure that the cloth is saturated.
The second thing to watch out for is related to the fact that it is a very slow cure. You may get the entire hull and fiberglass all wetted out, and it may look great. Then you call it a day and leave the shop. While you are gone, and before the resin can set up, the residual tension in the cloth or a bit of outgassing in the wood, can cause the formation of a bubble, or wrinkle, or a lifting of the glass somewhere. By the time you see it the next morning, it is too late to fix it without the repair showing. What you must do is check on it every 20 minutes or so for the first hour or two after you're done to smooth out any problem areas. Then, you can extend the intervals to half hours, 45 mintues, and full hours, continuing to check it periodically for up to 8 hours or more until the resin is hard enough to preclude any further unwanted developments.
Clearcoat's low viscosity makes it useful as a sealer for hardwood trim items such as gunwales, decks, and thwarts, although you have to sand the items again after the epoxy cures, before you varnish. And, as with all epoxies, you must sand smooth the last coat and varnish, because the UV inhibitors in the varnish are necessary to protect the epoxy from the UV in sunlight. However, the low viscosity and slow cure works against you if you try to use it for fill coats, so it definitely is not recommended for that purpose. I use West System or similar for the fill coats.
On my most recent canoe, I used MAS Low Viscosity Epoxy Resin together with MAS Slow Epoxy Resin Hardener. I have yet to get to use this boat, but so far, the epoxy seemed to be very easy to use. It is not quite as low a viscosity as the Clearcoat, but it's thinner than the West System resins. I found that I got all of the benefits of the Clearcoat, but with less tendency for the resin to soak in too quickly. And, it was still thick enough to work okay as the fill coat. Time will tell how well it works overall, after I have used the boat for a while, but so far, it seems very good all round.
When applying fill coats, I use one of the standard foam rollers sold at any of the places that sell the resin, but I saw it in half first. The shorter roller gives better control and easier application over the many curves in a canoe hull, and you get in effect two rollers for the price of one. As you apply the fill coats, after each batch of 4 to 5 shots of resin ( any more and it'll cook off in the can before you can apply it, getting dangerously hot in the process), you'll need to use a foam brush to gently tip off the freshly applied resin to get rid of the bubbles. For the inside of the hull, I try to get a good wet out coat and then leave off the fill coats altogether. Leaving the weave of the cloth exposed makes for a non-skid surface and saves weight as well.
To finish off the inside of the hull, I do one of two things. If the underlying wood has attractive colour and grain, I finish off with a couple of coats of matte finish exterior grade marine varnish. The only company I know of that makes such a product is Epifanes, and you will probably have to special order it - but it's good stuff, and produces a nice finish without much effort. The matt finish is easier on the eyes because it produces less glare, and it lets any fancy varnished trim stand out nicely in contrast. Alternatively, if the wood underneath is not all that attractive, I have used Sikkens Anti-Skid Deckpaint on top of Sikkens 2 Part Epoxy Primer. This has worked very well, and looks terrific. Incidently, the 'ugly' wood in the hull was made to look very nice on the outside of the hull by staining, which is covered in another article in the Builder's Notes. However, I do not recommend trying to stain the inside of the hull. It would be too difficult to get it even, and you might end up with water trapped in between the fiberglass skins, which could cause delaminations.
So, good luck with your fiberglassing. If you follow the tips above, I'm sure it'll be easier for you than it was for me on my first few boats!