How To Fiberglass (With Pictures)

My friend Kerry asked how do you fiberglass? I tried to explain in a facebook comment but, for those folks who are interested in a real answer, a bit longer of a post and pictures are required.

Fiberglassing, as a process, requires a fabric (the fiberglass) and a liquid (“epoxy”). Epoxy is a chemical that looks like maple syrup and hardens into plastic, like Tupperware.

Note: there are all kinds of uses for epoxy besides fiberglassing. My friend Bernie made me coasters from “epoxy pours” on top of flower petals.  I have four pieces are artwork in my parlor from Brittany and Julie made from dripping epoxy colored with pigment onto a canvass (or maybe they pour paint onto a canvass and then cover it with clear epoxy).

The utility of epoxy comes from the fact that it starts in a liquid phase and then, after mixing it with a “hardener”—another liquid shipped and stored in a separate bottle—it hardens into a solid. That allows you to pour the epoxy or paint with it in liquid form, and when it hardens it becomes a super strong material.

How strong? If you take two 2X4s and paint the tips of them with epoxy and stick them together, and let the epoxy cure, it will stick the 2X4s together and then if you try to smash the 2X4s they will break anywhere along the wood before breaking where they have been attached by the epoxy. Because the epoxy soaks into the fibers of the wood itself and turns the wood into a plastic.

Some bar tops have a hard, clear sheen to them, overtop of chemically-tinted copper, or wood or even pennies. The sheen comes from mixing a gallon of epoxy with a few ounces of hardener and then pouring it over the copper, wood, pennies, etc and letting it harden into a plastic not unlike plexiglass. El Loco has a bar top like that.

Above is “resin” and “hardener” from West System, a manufacturer of epoxy that began operations in the late 1960s, I believe, when fiberglassing was starting to become a thing. What I like about West System is that it comes with pumps that make it easy to mix because they measure out each thrust of the pump in an exact ratio for mixing. It is super important to mix the resin and hardener in EXACT proportions. Over the years I have mixed wrong and ended up hardening a $100 gallon of resin in the can because I put too much hardener in. Sometimes you literally have to use an eye dropper to mix the exact amount of hardener by ml or cc.

The thing about epoxy is that once you mix the hardener into it, you have a very short window that it remains liquid, called its “pot life.” Generally, at 72 degrees Fahrenheit, once you mix the hardener into the resin you have 2 hours to work with it. At 62 degrees you have 4 hours; at 82 degrees you have 1 hour; at 92 degrees you have 30 minutes, etc. Again, I lost over $100 per gallon cans of resin when trying to work with the stuff in the winter by heating up the resin on a hot plate before mixing the hardener in, and then—whoops, I must have heated it to 112 degrees because it hardened in 7 minutes, before I’d applied 1/25th of the stuff to the boat I was trying to water seal.

Another thing to keep in mind is that epoxy-creation is an “exothermic” reaction— as it happens, it gives off heat. Since it hardens more quickly the hotter it is, and also gives off heat as it hardens, it is likely to undergo a cascade reaction the larger the quantity of mixed material. In practice, if you mix hardener and resin and want to extend it’s pot life, you can stick the container in an ice bath (lowering its temperature through convection) or you can spread it out into a pan or film, delaying the cascade (self heating) chain reaction because the heat given off will be convected into the atmosphere rather than back into the epoxy where it would further speed the hardening process.

The first thing I did on my boat was to paint the entire bottom with epoxy to make it waterproof. In the old days (1970 through ancient history) wooden boats would be placed in water for a day, and they would leak, and then the water would be pumped out, and the boat would be waterproof because the wood had expanded via absorption to close small cracks. That only works if you can leave a boat in the water for more than a day. If you trailer an old wooden boat like mine, it dries out every time you take it out of the water and then leaks every time you put it in. Fiberglass boats don’t have that problem because they present a solid plastic shell to the water. Coating the bottom of the boat with epoxy fills small cracks and provides a much stronger protection from punctures. The epoxy soaks into the wood and turns it into a plastic.

Somewhat hard to tell but I have gone over the seams of the bottom of the boat with epoxy, which is kind of like painting with polyurethane. That won’t fill gaps, because the viscosity of the epoxy is water-like. To fill gaps requires the use of “fairing compound” which is a two-part compound (epoxy) which has stuff mixed with it to make it fill a greater volume of space (fill cracks) at the cost of strength. You mix the fairing compound from two containers which each have a Playdough consistency into a “peanut butter” consistency. (Yes, peanut butter consistency is the industry term for what you are aiming for as you mix the two products.)


The thing about the fairing compound is that it fills cracks. It’s like plaster or spackle when you’re doing woodwork. You can get close enough with your measurements and cutting and then fill the gaps and sand it after you’re done, as long as you are painting it. Same with fairing compound. Unlike epoxy it is easily sandable. Then you end up with a smooth surface. Make sure you coat that with more epoxy so it is waterproof and hard. Then you can fiberglass or paint. This is how people make pontoons — they take foam, cut it, fill in gaps with fairing compound and sand it until the whole shape is smooth to hands and eyes, and then they can finish with fiberglass.

Fiberglass itself is a fabric which looks like wedding dress material.

You buy it by thickness and weave pattern. The thicker it is, the more $100 per gallon epoxy it takes to “wet it out”. The different weave patterns flex and stretch to fit shapes more effectively and in ways that create more strength. I can’t remember exactly because this fiberglass was left over from a boat project from around 2012 but I think it is an 8 oz sheet with standard weave that makes a chris-cross pattern. I laid it over the boat as best as I could, taping down sides so it wouldn’t blow away and trying to create as few wrinkles as possible.

Once the fiberglass sheet is laid out, the only thing left to do is to wet it out with mixed epoxy. But that’s where the art and skill really come into play. You want to mix the epoxy in small enough quantities that it doesn’t harden in the pot. Even if you do that, it still eats about one foam paint roller every twenty minutes, so you’ll go through a lot and there is no way around it because the foam crumbles away leaving you with only the metal paint roller spine.

When finished, the epoxy turns the white fiberglass transparent, as long as you have used enough epoxy to wet out all of the fibers. Sometimes the underlying wood can suck up all the epoxy and leave a cloudy appearance. That’s why I coated the wood with epoxy and let it cure before I put the fiberglass on with another coat of epoxy.

Then you have to go around and 1) cut off all the fabric they you did not coat with epoxy and 2) use a utility knife/box cutter to cut out any wrinkles which hardened into the shape. It is impossible to avoid wrinkles in “complex” shapes that curve in two directions.

Then you have a shape which has been coated with fiberglass but it is no longer smooth because you have cut off wrinkles and there are ends of the fiberglass sheet that are not sealed. So you have to use more fairing compound to fill those gaps, and then sand that fairing compound after it has hardened after 2 hours, and then coat the spots with the fairing compound with epoxy.

At that point, you just repeat the last few steps, coating with fiberglass, letting it cure, cutting wrinkles, filling with fairing compound, sanding, coating with fiberglass, etc, until you have reached the desired thickness and firmness you are looking for.

On a boat you also have to coat the final product with a “gel coat” which is a two part epoxy paint—basically epoxy with pigment in it—although I don’t know why that is. Here is the bottom of the boat after the fiberglassing and with a coat of gel coat:

And lastly I coated the bottom of the boat with the same green “anti-fouling” paint (which somehow dissuades little mollusks from attaching to the underside of your boat) that the boat had when I bought her.

I almost forgot…there are no more superstitious people than sailors. I think it is related to the phenomena that keeps religion in fox holes. Anyhow my friend Kay came over as I was working on the boat and she was bound and determined to find a four leaf clover, which she did, and I painted it into the bottom of the boat for good luck (above).

That’s fiberglassing! It has a lot of more applications than boat building. Any surface repair job that is not completely flat benefits from epoxy/fiberglass knowledge. It bonds surfaces together more strongly than glue because it doesn’t merely affix surfaces but seeps into them and chemically combines them into the same object.