Resins for the clueless

So. You have done a little research at the ol' Youtube University, and you are thinking about starting some kind of composites project. ONE CATCH! You just realized that actually you have no idea what you are doing. Your precious youtube videos did a great job of explaining the minutiae of the higher level subtleties in sanding technique or fabric choices, but then you never got any foundational education about the very basics.

Good thing I'm here. Let's get beyond basic on resin.

The resin is the sticky stuff that hardens and holds the fabric in place. It is the glue. There are tons of types of resins, but there are two main families: epoxy and polyester. (There's vinylester too, and I don't even know how UV cure resins work, but this is supposed to be beyond basic, so the esters are all getting grouped together and everything else is getting ignored. Plus I don't actually know all that much.) When people talk about gel coat, they are talking about a polyester product 99 percent of the time. You can search for epoxy-based gel coats, and you might find a few options that sort of match that description, but it doesn't really work that way. To talk about why, let me tell you about the basic properties of these two families of resin.

First off, beyond basic, these are all two-component resins. Epoxies have a resin and a hardener, polyester resins have the resin and the catalyst (mekp). Epoxies use roughly 20-50 % hardener (every flavor has different instructions), polyesters use about 1-3 % catalyst. So basically polyester resins are perched and ready to cook themselves off, and epoxies are missing like half the ingredients until they're all mixed up. It should make sense then that epoxies tend to have much longer shelf lives, and polyester resins tend to solidify much more easily. Fun digression: if your epoxy crystalizes a little bit in storage, it probably got too cold, and you can probably use a hot water bath (120 F) around the exterior of the container to melt the crystals, kind of like honey. If your polyester or gel coat crystalizes, it's probably now a hazardous material that needs special disposal. But I'm no expert. Let's get back to the fun stuff; this digression is no longer fun.

With epoxy, if you double the amount of hardener, you just don't get the right cure, and could end up with a gooey mess. As far as I know, the process takes about the same amount of time no matter the mixture, although temperature matters. With polyester, if you double the amount of catalyst, the cure happens much faster and all kinds of stuff could happen. For either family, temperature is a really strong control. Hotter is faster, and either reaction produces its own heat so if you have too much of it all together it can accelerate itself. Everybody seems to know somebody who mixed up too much resin and had it catch on fire. It was Quinn.

So far, pretty much everything has been the same for both types. Here's the essential difference: polyester resin does not cure when it's in contact with the atmosphere. Epoxy does. There are probably more precise ways of describing that but if you care that much, go google an article written by an actual professional. What this means for your project is that if you lay up a layer of fabric with a polyester resin, most of it will harden just fine but the very last little skin layer will not be cured and will be a tiny bit tacky still. If you do the same layer but with epoxy, the skin will fully cure and in fact some little contaminants (amines and moisture) from the inside will float up to the surface and form a little slippery anti-sticky layer on the surface, which is called the blush. So, the essential difference: if you try to lay another layer on top, the polyester resin will chemically bond to yesterday's polyester resin, but the epoxy will not bond to yesterday's epoxy.

This is vital and can be used to massive advantage if it's part of your plan. This is why gel coats are polyester and an epoxy based gel coat doesn't really work. You can paint a polyester gelcoat into the interior of a mold and let it harden, so that if you put fabric on there it doesn't mess with the body of the gel coat, and that new layer will still be bonded on firmly. Gel coat is basically polyester resin with a bunch of special stuff mixed in. I think the origin of the name "gel coat" was that at some point in the distant past, it was a coat of normal resin which you would just let cure a little until it turned to a gel before applying it. You could totally do this with plain polyester resin. If you try to do this process with epoxy, it will not really work right no matter what additives you have. You can kind of get something to work but you tread a fine line. You would have a very time sensitive process where you would be trying to get the next layer on there after the first layer had started to solidify, but before it had solidified enough to seal the surface. With the epoxy version, there would be no real difference between the interior and the skin. With the polyester version, you can do a layer and let it cure for anywhere between a couple hours and a week or two, and the bulk is solid but that skin is still ready to bond to. 

When it's time to seal a polyester resin or gel coat, you just need to isolate it from the atmosphere. The common way to do this is to mix in some wax and styrene, so that as it cures, the wax floats to the surface and give you a sealing layer. You can also paint pva (polyvinyl alcohol, mostly used as a mold release film) on top or even maybe just wrap it in plastic, although you'll have differing amounts of success and each method has side effects. If you're buying gel coat, read the fine print. Some gel coat is designed for repairs or painting on the exterior of ship decks, and has wax mixed in so that the surface will seal itself, cure tack free, and nothing will bond to it. This is called finishing gel coat. Some gel coat is designed to go into a mold and have new layers bond on to it. This is called laminating gel coat (this is the default, so if they leave it out of the name or call it tooling gel coat or that kind of thing, assume it falls in this class). If you use the wrong one, you're going to have a bad time.

So with all this description about how you can bond on to polyester, why use epoxy? There are a couple reasons. The most mundane one is that polyester is stinky and toxic. You need a mask whenever you use it. Epoxy is much more tame. If you get it in your skin you might get a rash. A more substantial difference is that polyester tends to shrink as it cures and epoxy tends to be stronger than polyester. So the usual procedure, at least for building kayaks, is to build your mold with polyester, and then build your boat with either epoxy or polyester. There are exceptions but this is the easier way.

So the usual procedure, which I've written about before, but I'll re-write now with an emphasis on resins, is as follows. You prepare your plug (your pattern, your original, whatever you want to call it) with some mold release so that neither type of resin can bond to it, then paint on some gel coat. You paint that mostly like paint. It can be really non-viscous, so that there is basically no chance of any air pockets or bubbles getting trapped against the surface. Then this layer hardens, but you can proceed to lay reinforcement fabric on top of that, one or two layers at a time to avoid shrinkage. (Or you can do two or three thin layers of gel coat before adding any fabric, since thin layers will have fewer bubbles.) Once you have your mold finished, you get it all waxed up and you're ready to build boats.

For boats, you probably have different objectives. Weight and stiffness probably matter a lot. Molds are often pretty thick for a variety of reasons, but boats are often as thin as possible. Epoxy is a common choice. Your decoration options are a little tougher but you'll probably get a stronger lighter epoxy boat. Polyester is still a fine choice. If you know what you're doing you can still get a very strong and light boat with polyester, and depending on your process you might want some of the advantages of a polyester system.

The one annoying thing, or at least one of the many annoying things, is that polyester and epoxy do not get along. In particular, the blush on epoxy will mess with the cure of polyester. And, even though it seems so promising, epoxy does not bond to the tacky surface of polyester (it does have a mechanical hold but not a true chemical bond). So doing a half and half kind of construction is not going to work. (Side note: Gui-Gui apparently uses a vinylester gel coat that can bond to an epoxy laminating resin, but he was very sparse on the details and this is right on the edge of being a trade secret.) Obviously, once either type of resin is fully cured and cleaned, it's chemically inert, so you should be able to repair anything with anything. If in doubt, though, like if you are applying a seam to a boat that is only a few days old, better to keep the same kind of resin. Another annoying thing that I learned the wrong way is that polyester resins and gel coats will dissolve styrofoam.

This little collection of tidbits is far from a real introduction, but I really wanted to set straight the difference in how the two resins cure. Writing the blog article helps me crystalize my own thoughts. Hopefully it is at least a little helpful to you as well, dear reader. With this basic bit of knowledge, you should at least have a starting point for knowing what to google if you want to start truly educating yourself.

My other composites articles:

Introduction to Composite boatbuilding

Composite boat repair for the clueless

Building the Slug

or maybe see if there's like a label that you can click on down below here, I don't know.