To build the Mayfly, its components get glued and screwed together. The wood screws aren’t rocket science, although the ones that remain permanently should be something rust-resistant, like silicon bronze ($$$, yikes!), or in my case, cheaper stainless steel.
The rocket science part is associated with the glue. Michalak says wood glue will do, but that epoxy is better. I’ve used epoxy on my prior kayak, and I agree. Its PROS include high strength, durability, and longevity. It can also be adapted to a range of uses, from bonding materials to protecting surfaces to filling voids to fairing irregular features. The CONS include its price, potential health effects, having to work in batches, and in some cases, surface residues that must be removed before epoxy surfaces can be finished. More on health and safety in a moment. Here’s the essential info on epoxy. And, yes, they make rockets using this stuff.
Marine epoxy comes in two parts, a resin and a hardener. Multiple hardeners are usually offered for a particular resin, and yield relatively fast or slow cure times. They get mixed at a manufacturer-specified ratio, which must be strictly observed. Once mixed, you have a certain amount of time until the mixture begins to solidify and becomes unworkable. In that time, you must add any needed fillers according to the task at hand, then finish the task. The length of working time is referred to as the pot-life, which varies greatly by the ambient temperature and the hardener you selected. Cold temperatures slow the curing process – higher temps speed it up. Once applied, the material goes from a liquid to a soft-cure stage to a hard final cure – the first change in minutes, the second in hours or even days. Working with the stuff after its hardened is almost impossible, so doing the job right the first time is important. WEST SYSTEM, a premier supplier, has nice a write-up of the cure process at http://www.westsystem.com/ss/epoxy-chemistry/. The U.S. Composites epoxy I’m using has a 3:1 mix ratio, medium cure time, and seems to have about a 15-20 minute pot-life.
Epoxy is adaptable to a wide range of uses, in part, because of the fillers that can be mixed into it. For filling seams, Wood Flour (finely ground sawdust) is mixed into the combined hardener and resin until it reaches a peanut-butter color and consistency. For bonding wood, Fumed Silica is mixed in until it looks like translucent mayonnaise. Mixing phenolic microballoons into epoxy makes it a fairing compound, a material that can be sanded – think of automotive body filler, only much stronger and totally waterproof. Mixing graphite power is supposed to make the final finish tough and more slippery, useful for boat bottoms. Unthickened, epoxy protects surfaces by forming an impermeable coating that can be beautifully finished – my kayak is made of mahogany that is covered with fiberglass cloth set in epoxy that was sanded and varnished. In every case though, epoxy requires a lot of mixing , minimum 2 minutes per batch, plus time to mix in any fillers.
For all of its advantages, I’ll be using this stuff sparingly because I am allergic to dermal contact with it. It can cause reddened, heavily swollen skin, a condition that is NOT FUN. That means I’ll use it to set critical load-bearing joints on the Mayfly, and fairing out the cheap plywood I’m using, but I’ll use other bonding agents for less-stressed structures above the water line. And I’ll be upwind of the stuff at every opportunity. Safety gear always includes vinyl gloves or better, protective clothing, and a respirator if you cannot work upwind.
If you are planning to use epoxy, consumable items make it easier. Disposable cups and tongue depressors or plastic spoons are needed for mixing each batch and working with the goop. A big box of vinyl gloves is needed, and soft pliable spreaders make surface coating easier. Zip lock bags with a small corner cut off may be be used as makeshift bead applicators.