This is Part 2 in the series where we will start with applying fiberglass over the wooden mold, frame, and structure. If you haven’t read Part 1, I will encourage you to see the previous Blog Post titled “How to make a Fiberglass Deck Box for your boat – Part 1”
As we do our preparation work for the fiberglass, we want to make the box as strong as possible. One of the things that can weaken fiberglass is a 90-degree corner. If you think of fiberglass material as many fibers of glass, you can see how bending them into a sharp corner can cause the fibers to break in that spot. Think of taking a piece of seaweed (nori) for making sushi. It is dry and crunchy, and if you held a piece and folded it at a 90-degree angle, it would crack. But if you roll it around a radius of a California roll, it stays stable and doesn’t break. That is because it is allowed to make a gradual bend. This same theory can be applied when you think of fiberglass. So what do you do if you have a square wooden box? I thought you would never ask.
To create a smooth radius in the corners for application of fiberglass cloth, you create a fillet. (Not a filet like a delicious steak, but a fillet or radius. This is a fancy work for smearing some thickened resin into the corner and smoothing it over. We can quickly take the mystery out of this.
This is the first time we will be mixing resin in this blog series, so let’s spend a few minutes on this foundation topic. You have many choices when jumping into a project like this. You can use Epoxy, Vinylester and Polyester resins as your base. They vary in price with Epoxy being the highest and Polester being the least expensive. For any project where you are going to be laying fiberglass over wood, I strongly recommend using Polyester resin. All three will create an excellent bond to the wood. However polyester is the cheapest. (About 35% of the cost of doing the same job in Epoxy.) Polyester resin is a 2 part material consisting of the resin as well as a catalyst. The catalyst is MEKP-925. This stands for Methyl ethyl ketone peroxide, and it is pretty nasty stuff so be sure to protect your eyes and skin when using it and follow the precautions called for by the manufacturer.
For the remainder of this blog post series when I refer to fiberglass or resin, I will be speaking about polyester resin for the sake of clarity and ease of writing.
You can buy Polyester resin at Amazon (And we have a link at the bottom of this page for all the supplies we used), but often you can get this at your local chandlery as well. I have seen this for about $26 per gallon, and at that price, the threw in the catalyst as well.
I mixed too much, and it kicked before I was done
The brand I have called for a mixture of 4 CC’s per 16 ounces of resin. You want to start by pouring the resin into a mixing cup up to your allotted amount. Helpful hint. Don’t use a red solo cup for this task, it will melt the cup, and you will have a horrible mess on your hands. We use plastic mixing cups designed for this, and they have markings on the side for easy measurements. Like any resin, you only have so much time before it begins to cure so only mix what you know you will use in a 15-20 minute timeframe. The hotter it is, the faster it will “kick” or start to cure. The higher the % of catalyst the faster it will cure as well, but understand if you put too much, it will prevent it from setting at all. The measurements for this should be very accurate.
We use a catalyst squeeze measuring cup for this task because it allows us to accurately measure 
catalyst in small quantities of 2 CC’s. Once you squeeze the correct amount into the pouring chamber, you can pour it into your resin. Once poured you are now racing against the clock. Mix it thoroughly for a full minute using a small stir stick. I like to use a wooden popsicle stick, but for larger batches, you can get a mixer that hooks to a drill. I have also seen people use an old kitchen hand held mixer with only water beater blade installed.
Once we have our resin and catalyst mixed, it is now time to thicken the material to a Peanut Butter consistency. Resins are described by food textures when it comes time to thicken them. For 
example, you can have Ketchup, Mayonaise and Peanut Butter consistency. Each is achieved by adding more filler material. I have chosen to use Micro Balloons because they were readily available and also because they do double duty as an excellent fairing material. A gallon of micro balloons weighs just a few ounces so if it windy be careful this stuff will blow away. I add it by the heaping tablespoon full and mix it after each addition. It will take a cup or cup and a half or so of filler for 8 ounces of resin. You can determine this as you go and by what surface you will be applying it to. Because I was going to be applying it to vertical surfaces, it was important to be relatively thick.
The best way I have found to apply this to the corners is to use a small spreader or popsicle stick. The back of a plastic spoon can give you a great radius for smoothing the corner and the higher the angle you hold it, the sharper the corner will be. The flatter you hold the back of the spoon against your surface, the broader and shallower the turn. Practice you first fillets in an inconspicuous space. My first ones were pretty disastrous looking and would require more sanding later, after making 4 or 5 of them I was efficient at it and able to create good looking ones without wasting much resin.
The next blog post will be all about laying the fiberglass over the wood now that we have a nice smooth inside corner for the cloth to settle in.
There is a full list of materials and tools we used on this project (HERE) along with links to most of them on Amazon for your convenience.
The first installment of the video series covers the topics in Blog post 1 and two on this subject.
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