Epoxy and Fiberglass

On most boats, epoxy and fiberglass are core parts of the construction. On our “cold molded” boat, the boat is created by laminating sheets of plywood with epoxy over a mold (the ribs in our case), and then putting fiberglass on the exterior. This is different from regular “wood cored” construction, which puts fiberglass on both the interior and exterior to provide strength, with light wood core for stiffness. Cold molded saturates the wood core itself at each layer with epoxy, and derives much of it’s strength from that.

All that to say, fiberglass and particularly epoxy are important to the construction of our boat, and materials that we use a lot. Epoxy is a material that’s easy to get lazy with, and it’s important to always be keep aware when using it.

Epoxy

A Warning

Before going any further, strongly consider how you will remain safe from epoxy. If you handle it too often and it touches your skin, you can develop a sensitivity to epoxy and become allergic to even the smell of it. As you have to use it frequently working with boats, this is a major issue. In the beginning we were reckless, and got it in our hair, face, hands, and arms. This sounds like it would be easy to avoid, but when epoxy has “kicked” too quickly (started to harden), it’s easy to move quickly to finish the job and end up spilling and mopping everywhere. We now always wear eye protection, a mask (for working with fillers), long sleeves, and are very careful with gloves.

As a second warning, do not get lazy with your mixing ratios! Epoxy only hardens completely if mixed by the correct ratios in weight or volume. We normally mix by weight, and in small amounts this can difficult to get right. For that reason we make slightly larger epoxy batches, and simply waste the extra epoxy. The tolerances for the ratios aren’t that tight, and there is some wiggle room, but epoxy jobs aren’t something you want to redo.

Brands, Resins, and Hardeners

They are all pretty much the same thing. Our boat was built with “System Three”, and they and “West Systems” provide the most information online. West Systems can be purchased locally where all others are difficult. Go with West if using small - medium quantities because it’s available. Otherwise Raka has good reviews and is cheapest if using a large amount. You can also consider “Vinyl Ester” depending on the job, but that’s outside the scope of this page. We will assume West Systems as it’s the most common and what we are familiar with.

“Resins” are the “Part A” of epoxy. It’s the stuff with high viscosity, which is mostly clear. Different brands have different levels of viscosity options, but West Systems has only one viscosity. Thinner viscosities are preferred if you want to saturate wood for instance, where a thicker viscosity might be preferred when you need a thick substance to bond things together.

“Hardeners” are high viscosity and normally have a color tint to them. West Systems has two hardeners. The fast (205), and the slow (206). On most jobs we’ve grown to prefer using 206 even though it takes longer. Complicated epoxy jobs tend to spill on the deck, and it’s nice being able to wipe up the excess. The 205 hardener is only used when we need to fill holes in the deck specifically at this point. If you want to dial in the times even further, you can always mix the slow and fast hardeners.

Fillers

Fillers are added only after the initial mixing of resin and hardener, and different fillers are used for different jobs. “Talc” provides a smooth finish and is best for non-structural applications like fairing and cosmetic repairs. It adds minimal strength but is easy to sand. “Colloidal Silica” adds significant strength and is what we use most often for bonding. You can control how thick the epoxy is by how much thickener you put it in, and with enough filler it can be attached to vertical surfaces without sagging. “Microballoons” create a lightweight, easily sandable mixture, which is good for fairing. They add minimal strength and are best for non-structural, cosmetic repairs.

Epoxy Uses

Our most common use of epoxy, is drilling holes in the deck and using it to protect the core from water. If we are screwing things into the deck, the process looks like below. If we are simply running wires through the deck through cable glands, we ignore the sealant and create a “epoxy core” so that no water can touch the wooden deck core. The general idea is make sure epoxy is the only thing that leaks can touch, and that the core is completely protected.

  1. Drill regular hole first for screw
  2. Use stepped drill bit to double the size.
  3. Tape under each hole, fill each hole with unthickend epoxy for 5 minutes to saturate, then cut tape and let drain out.
  4. Tape under each hole again, then fill each hole with a syringe with thickened epoxy. Let harden overnight. If hole is smaller, drip thickened epoxy in with a toothpick.
  5. Redrill holes at original size, and use countersink bit to create more area for sealant.
  6. Use butyl tape or sealant (sealant preferred for us) to clamp it all down.

Another common use for us is creating permanent “studs” in thick material, so that deck hardware can be easily be removed. Our largest use of this was for the radar pole when we didn’t have room for a through bolt in the deck. Directly below the deck was 5 inches of a solid piece of wood. In order to create studs for the radar pole flange, we drilled holes 1/2 inch in diameter and 4 1/2 inches deep. We then “keyed” (roughen up all the sides) the inside so there was more for the thickened epoxy to “grab”. We then filled with unthickend epoxy to seal the hole (like above), used a syringe to suction it out, and then filled the hole with 3/8ths stainless steel studs cut to size, and lots of thickened epoxy. While not as strong as a through bolt, this process is remarkably strong and West Systems documents it’s use in wind turbines. For more specifics, look at this “Epoxy Works” page.

Fiberglass

While we’ve covered a lot of uses of just epoxy, it’s often thought of in coordination with fiberglass. The basic process is to grab fiberglass of various thicknesses and patterns, and then to “wet out” the fiberglass with epoxy. Normally done with a small metal roller or a paint brush in a pinch. On our boat we’ve used fiberglass pretty rarely, as it’s only something we add for stiffness if something has been dented or pierced. We’ve used it occasionally to repair rot or add new blocks for instruments on the mast head, but we won’t get real experience until needing to make a major repair. However below is what we’ve figured out so far.

Fiberglass Fabrics

There are many kinds of fiberglass fabrics, but the most common ones we use are 6 ounce “plain weave” and 12 ounce “biaxial”. The 12 ounce biaxial is stronger, but harder to work with compared to 6 ounce plain weave. This means it’s harder to put around curves and be uniform. If you need structural support use biaxial, but simple protection can be achieved with 2 layers of 6 ounce plain weave.

There are also “mat” fabrics or “chop strand”. These are the easiest to apply to curves and corners, but normally they only work with polyester/ vinyl ester and don’t provide much strength. However, they do make epoxy mat fabric, and this can be used with epoxy. For jobs that don’t need to resist much impact, we often reach for this and it’s sufficient. Even 6 ounce is a little tricky to work with and cut, and this can be added almost like cotton candy.