It can be a real challenge to achieve a totally clear, transparent finish with fiberglass and epoxy. I personally have never run into any serious problems, but I have heard of disasters such as mixing errors where the epoxy doesn't cure, using coated fiberglass that won't "wet out", trapped moisture causing cloudy resin, trapped bubbles, epoxy kicking off too soon, failure to wet out due to starving the glass, subsequent epoxy layers pitting or not adhering due to surface contamination or amine blush... the list goes on and on! It is important to be familiar with the epoxy system that you are using, and know that it works with the fiberglass you chose. Control of the temperature of the resin and workshop is also very important.
After our board was completely sanded, we sealed the wood with a thin layer of epoxy. The purpose of this was to saturate the wood before putting on the first layer of glass. Some builders consider this step unnecessary, but I always do it. If you leave the wood unsealed when applying the glass layer and wet it out, the wood could soak up too much of the epoxy, "starve" the glass and result in dry, blistered areas where the glass didn't wet out fully. Saturating the wood prevents this. Different wood species will soak up different amounts of resin. There is a lot of variation in color and density in western red cedar. Not surprisingly, we noticed that some strips soaked up a lot more resin than others.
When sealing the wood, keep the both the wood and epoxy warm. Heating the epoxy keeps the viscosity down. Although the higher temperature will accelerate curing, don't worry: with slow hardener you still have plenty of time to work with it before it kicks off. After applying the epoxy, allow the shop to cool. As the temperature drops, the wood will soak up the resin. You definitely do not want to start with a cool shop and warm it up after applying the resin. This could make the wood "off gas" as it heats up and create bubbles in the resin.
I use MAS Low Viscosity Resin and Slow Hardener. You will need at least one half gallon of resin and one quart of hardener for this board. (You may be able to get away with only using that if you are more careful than we were.) We placed our epoxy containers in a tub of warm water and used an electric heater to warm the shop. Back when I was building my cedar strip kayak, I used a kerosene heater to get the whole shop to at least 80 degrees F. Another effective way to heat the wood is to shine some hot lights right on the wood surface or, if you are building a kayak or canoe, put lights inside the boat.
We spread the epoxy on in a thin layer with a spreader and evened it out with a brush. You want it as thin as possible. We waited 24 hours for it to cure before doing the other side.
Working with Epoxy
The big hazard associated with working with epoxy is the development of a severe allergy. Direct skin contact with the hardener is usually the cause. It usually manifests as a contact dermatitis but can also include respiratory difficulties. It can be severe enough to keep people from ever working with epoxy again!
When using epoxy, work in a well-ventilated shop and use a respirator with an organic vapor cartridge. Wear eye protection. I wear nitrile gloves and avoid getting resin on my skin. Applying a layer of barrier cream to your hands and arms provides additional protection. If you do come in contact with the resin, use a waterless hand cleaner. You can wash it off with white vinegar, although I’ve heard that dissolving it in vinegar makes it more easily absorbed through your skin.
Another concern: you may have heard of a substance called bisphenol A, or BPA for short. BPA is a weak endocrine disruptor, which can mimic estrogen and may lead to negative health effects. High levels of BPA are associated with heart disease, diabetes, elevated liver enzymes, brain tumors, impaired immune function, and ovarian dysfunction, and erectile dysfunction. BPA is used in the plastics that coat almost all food and beverage cans, hard transparent plastic food containers, and reusable sports water bottles. Our main exposure to BPA is through food contamination, but it is easily absorbed through inhalation or the skin, for instance through handling thermal paper printed receipts, tickets, and boarding passes. In September 2010, Canada became the first country to declare BPA a toxic substance.
Epoxy resin IS BPA!
Wetting Out the Glass
After both sides of the board were sealed, we unrolled a layer of 4 oz uncoated plain weave fiberglass intended for clear coating. You should buy it rolled and never folded. For our 12 foot board we bought a 26 ft roll, 50 inches wide. We made a few cuts at the corners of the tail and around the nose so that the fabric would drape smoothly around the sides. Use a dry paintbrush to smooth out any wrinkles. Make sure the brush isn’t shedding any lose bristles on the glass. I would warm up the shop and epoxy as I described previously. The high temperature lowers the viscosity of the epoxy. This is especially important when trying to wetting out fiberglass to get a transparent finish.
I use volumetric pumps to ensure the proper ratio of low viscosity resin to slow hardener (2:1). I discard the first squirt from the pumps if they haven’t been used recently to eliminate any air trapped in the pumps. Measuring out the volumes in separate graduated cups is going to be less accurate, especially when you take into account that you have to mix the two, and there will be either resin or hardener stuck to the side of the one of the cups. This is also the reason why I don’t measure resin or hardener by weight. The volumetric pumps make it easy to mix up even small volumes in a single container as well.
Pour the epoxy over the glass starting in the middle of the board and spread it out to the ends. Spread the epoxy in a thin even layer with a spreader, working it into the glass. Use a paintbrush to wet out the sides, again being careful not to leave any loose bristles behind. You can clean uncured epoxy with white vinegar, but you really don’t want to have to bother with that -- just use a disposable chip brush and throw it away. I cut the bristles short so they will be stiffer and work better with thick epoxy. You want the glass to be transparent from all angles. You may end up with areas with multiple small air bubbles. These may respond to the application of directed heat with a heat gun.
For an excellent example of how to wet out glass with epoxy, check out this video clip from Guillemot Kayaks. Allow the first coat to fully cure (about 24 hours, depending on the ambient temperature) then cut the excess fiberglass hanging over the edges with a razor or sharp knife. I use a scraper to feather the edge of the glass.
You will still see the texture of the glass but it should be transparent. It’s difficult to fix any areas that did not completely wet out at this point. You could carefully cut, scrape, and sand the glass off and repair the hole with a patch of new glass, but sanding into the wood will result in a discolored area.
Turn the board over and repeat the process on the bottom. The glass will overlap on the rails, so the rails will get two layers of glass total.
The strategy here is to apply just enough epoxy to wet out the glass. The weave will be filled by one or two additional coats of epoxy after the first coat is cured. You might be wondering why can’t we just lay the glass on bare wood and apply a single thick layer of epoxy on it to accomplish all three steps at once: sealing the wood, wetting out the glass, and filling the weave. Some builders claim to have gotten away with that. I’ve tried it and discovered that if you apply a thick layer of epoxy while wetting out the glass, the glass will actually float on top of the epoxy layer and create big wrinkles in the glass. I had to go back and scrape out as much of the epoxy from under the glass as possible, resulting in a lot of wasted resin. You should use as little epoxy as possible when sealing the wood, wetting out the glass and filling the weave. Extra epoxy does not add strength, only weight.
When the glass layer has cured, apply another one or two coats of epoxy, just enough to fill the weave. The subsequent layers may take longer to cure, especially in cold weather, because they are thinner. When they are completely cured, sand the whole board again with the random orbital sander using 80 grit paper to smooth and fair the surface, then again with 120 grit paper to smooth over the scratches from the 80 grit. Avoid sanding through the glass -- stop if the weave starts to show. And don't sand incompletely cured epoxy. The dust from uncured resin is much more sensitizing than cured resin. When not fully cured, sanding will take much more time and effort, and the uncured resin will clog up the paper, and clump up into small white grains. When fully cured, the dust will be very fine and the sander will cut through the epoxy much easier.
Be especially careful with the sanding dust: it contains both epoxy and glass fiber. Getting it all over your skin can make you itch. I wear a respirator, gloves and long sleeves, and immediately take a shower and wash my clothes afterwards.