How to Use Fiberglass Resin

fiberglass techniques

Fiberglass microfibers allow you to control the viscosity of the resin and is a very important fiberglass technique.

Fiberglass can be messy and hard to work with for the beginner. I know that I made a lot of mistakes and messy work when I was first learning how to use fiberglass resin. The video below shows you the fiberglass techniques that I use when doing repairs on my boat.

Many people are familiar with mixing two part epoxies. The instructions for mixing them are clearly labeled on the container. Less well documented is the use of additives. One tip on how to do fiberglass properly is to a use filler like 403 microfibers. These kinds of additives allow you to control the viscosity of the resin and allow you better control when applying the resin to the fiberglass.


In this repair, I’m not using any fiberglass other than the microfiber additive. This is fine for these kind of small blemishes and small patch jobs in the deck and hull of a boat. For bigger jobs though, you’ll want to use more fiberglass with your resin.

Fiberglass Techniques

west system fiberglass repair

In this case, I mixed microfibers into the resin until it was the consistency of peanut butter. It doesn’t need to be this thick when working with cloth or mat.

There are three main fiberglass materials: cloth, mat, and woven roving. I hardly ever use woven roving as it’s used for very heavy duty work. I use mat for building up material, usually when trying to mate two pieces at a 90 degree angle. I then put cloth over the mat to make a smooth finish.

Using an additive is a good idea, no matter what type of fiberglass material you’re using. Fiberglass microfibers and colloidal silica fill in gaps in the materials, making it less brittle and significantly stronger.

Finally, fiberglass mat is designed to wet out with polyester resin and has a harder time wetting out properly with epoxy resin. Making your resin thicker with the additive gives it more time to soak into material. These are hard won fiberglass techniques that I use to do fiberglass properly.

West System Fiberglass Repair

I’m a big fan of West System epoxy and fiberglass products. The products are time tested and the company is dedicated to making it easy for beginners to use their products. Below are the products that I used in the video above:

105 Epoxy Resin

The epoxy resin is the most basic ingredient in any fiberglass repair. Polyester resin is generally used in the construction of a boat, but epoxy resin is widely regarded as the best to use for repairs. West System epoxy is widely regarded as one of the best epoxy brands on the market.

205 Fast Hardener

Hardener is the second part of a ‘two part epoxy’. The working time with fast hardener is about 15 minutes. It works down the temperatures as low as 40 degrees F. It can used up to 70 degrees, but really after 60 degrees F, it’s a good time to switch to a slow hardener.

206 Slow Hardener

Slow hardener is for use in temperatures above 60 degrees F. Other than that, it’s really not any different than fast hardener. Like it’s name implies, the working time with slow hardener is longer than it is with fast hardener for any given temperature. I don’t really think of it like that though. I use 60 degrees F as the threshold for when I switch between the two. It’s good to have both on hand.

403 Microfibers

403 is a general purpose adhesive filler. This is the easiest filler to work with when first learning. It mixes easily and applies well in almost all situations. 406 colloidal silica is stronger, but is much more difficult to mix properly. (In my experience)

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4 Responses to “How to Use Fiberglass Resin”
  1. Dineberu says:

    Good work….

  2. Nick says:

    Working with microfibers… also a great way to get lung cancer.

  3. Jessica Hunt says:

    What would cause the resin to ‘crakle’ as it drys? This happened on the third, and what would have been the final layer.

    • Chris says:

      Resin will ‘crackle’ when it hardens rapidly. This usually happens to epoxy in the cup. As it hardens, it generates heat. The heat causes it harden faster. This is known as ‘thermal runaway’. This behavior is a clue that you’re mixing up too much epoxy at once. Mix it in smaller batches and get it layered on before it starts to harden like that. It also happens when you layup too many layers at once. Do a layer, give it an hour to set up. Then do the next layer.

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