The Montauk Catamaran Company Chronicles 12/02/14: Epoxy Glue

(The Montauk Catamaran Company Chronicles is an ongoing series of posts detailing the construction of Mon Tiki Largo, a James Wharram Designs Pahi 63 MkII. The author’s current boat is the catamaran Mon Tiki, a JWD Tiki 38, which he built in 2012 and currently operates as a daysailing charter in Montauk NY. You can see all the posts in this series by clicking here.)

The author’s daughter spreading epoxy glue using a notched squeegee.

As mentioned in the introductory post, one reason for blogging the construction of Mon Tiki Largo is to (re)capture various details that seemed so much a part of our daily routine when building the catamaran Mon Tiki that surely they would never be forgotten, and yet have been.

For example, one of the first things I had to re-develop was my epoxy glue recipe, and then refine it for the conditions and quantities we’re working in on Mon Tiki Largo.

So then, for starters, we are using Raka Resin’s Medium Viscocity Resin and their Cold Weather hardener. The mix ratio for this combo is four parts resin to one part hardener, and this is the form it’s used in for encapsulating and laminating with cloth (usually glass cloth, but also polypropylene, carbon, kevlar and other fibers). But there are lot of other things you can do with epoxy, and many of these things are accomplished by the addition of “fillers”.

Fillers can be (off the top of my head)  milled glass fibers, glass microsphere, carbon powder, aluminum powder, phenolic micro balloons, chopped glass fiber, fumed silica and wood flour. I’ve even heard of people using talc, diatonatious earth and wheat flour. What filler you use and how much depends on what you want to do with it, but for now let’s talk about glue.

One of the beauties of epoxy as a glue is that it’s considers a “gap-filling” glue, and it’s not unreasonable to expect epoxy glue to correct sloppiness of joinery up to 1/8th of an inch. This in turn is why using unthickened epoxy is not considered best practice. Unthickenned epoxy would run out of the joint, which would mean we wouldn’t get the benefit of it’s gap-filling properties.

Fortunately there’s something called “fumed silica”. Without going into the details of what it is or how it’s made, fumed silica is a glass product that is so light by volume that we refer to it in the shop as “clouds”, as in “two scoops of clouds per pint of epoxy”. The more clouds you stir into your epoxy, the thicker it gets, and the common measures of thickness are made by comparisons to foods: honey, ketchup, mayonnaise, and peanut butter.

Epoxy thickened with fumed silica makes a perfectly good glue, but it can be made even stronger with the addition of another additive: milled glass fibers.

Again, without getting into the specifics, milled glass fibers are sort of the opposite of fumed silica. A pound of fumed silica is about 2-3 gallons by volume; a pound of milled glass fibers is about a pint. (In the shop we call fibers “the heavy stuff.) What these fibers do in the glue matrix is transfer load at the microscopic level and make a much harder, stronger glue. So whenever we’re building any sort of high-load bearing piece, we add milled fibers.

To make glue for putting the akas together, a situation where we’re gluing flat surface to flat surface, we want a glue that’s runny enough to spread well over the entire surface, but thick enough to not run out of the joint when it’s clamped. In fact, in epoxy joinery there is a phenomena called a “glue starved joint”, which happens when the glue is not thick enough, not enough is used, and/or two much clamping pressure drives the epoxy out of the joint. That’s right, with epoxy, sloppy carpentry and generous amounts of glue is actually better than hair-line precise joints and high clamping pressure. For gluing the akas, the glue we make has the consistency of ketchup.

The next consideration is how much glue to make.

You see glue has to be mixed as needed. You can’t mix up a bunch and leave it lying around because a big mass of epoxy will heat up and catch on fire. What we arrived at is one lamination of 1×6 over the full length of the aka requires about 10 oz of epoxy (8 oz resin, 2 oz hardener), 32 oz of fumed silica, and 2 tablespoons of the heavy stuff. This makes a glue that is easy to spread, but stays where it’s put.

Now a Montauk Catamaran Company Boatshop trick, and one I had forgotten.

The usually way is to mix the resin and hardener, then stir in the additives, then trowel this out with some sort of spreader. The Wharram suggestion is to trowel into a plastic bag and use like a pastry bag. But we go one better.

We get 1 gallon ziplock freezer bags and open them over a 1/2 gallon mixing bucket. They just fit with the zipper closure on the outside where it won’t get fouled with additives or epoxy. The various additives are put into the bag. (And if you really want to get slick, you can pre-fill these bags and have them waiting at the ready for a big session!)

The proper quantity of resin and hardener is mixed in a separate container. One it is thoroughly mixed, it is poured into the zipock with the additives. Then the bag is sealed and mixing is accomplished by kneading the bag. Once this is thoroughly mixed, a corner of the back is cut and the the glue is depended pasty-chef style. This method does a couple of things;

Bucket mixing with additives tends to become a big mess, and  just like a jar of peanut butter, you can never get all of the product back out of the bucket no matter how much you scrape. As subsequent batches are made, the mixing bucket gets more and more fouled until finally it has to be abandoned. Probably along the way your gloves have become fouled and you had to change them, probably at the most inconvenient time possible. And when you come back in the morning to try and “break it out” to use it again, the same properties that make for good glue make for hard cleaning. In our method we use fewer mixing buckets by an order of magnitude, and more of the epoxy we make makes it onto the project. (Again, if you really want to get slick, keep a rolling pin handy, or big dowel, and you can get virtually all of the product out of the bag and onto the project.)

Working in bags also lets us mix larger batches. In a mixing pot, four ounces of resin is about the most I can manage. The resin starts to heat up and will run away (smoke and even catch fire) if you can’t get it mixed and dispensed fast enough. On Mon Tiki our epoxy master was doing pint shots to keep us supplied when we were fairing. the hulls. (Fairing compound is a different recipe I will address on another day.)

One other thing.

Before we spread out the glue we also prime both surfaces with unthickend epoxy. My theory is that the unthickened epoxy penetrates more deeply into the wood for a stronger joint. It also prevent the wicking action of the wood from drawing epoxy out of the glue and leading to the dreaded glue-starved joint. When the surfaces are mated with glue, the glue and primed surfaces will bond at a chemical level.

So there you have it, epoxy glue. Fairing, just mentioned, is another thing we do with epoxy. Also filleting and laminating with cloth. All will be addressed, probably multiple times, in future installments.

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