Boat Building Fiberglass Resin – Polyester vs Epoxy

Epoxy vs Polyester

Basically, there are only two types of fiberglass resin.  There is polyester and there is epoxy.  Both are used in boatbuilding. 

Most production boats a constructed with polyester resin.  The manufacture has a mold that they coat with a hard-glossy film called Gel Coat.  After this they spray a chopped fiberglass polyester resin mixture inside the mold. Some companies hand lay the fiberglass soaked with resin instead of the spray method. When removed they have a sturdy structure with a glossy Gel Coat finish.  I might also add that the weight of the structure is a lot greater than wooden boats.

Most wooden boats are covered with some sort of fiberglass resin and fiberglass cloth.  The technical term for this is Cold Molded.  The end result, if properly constructed, is a lighter boat but just as strong or stronger than a factory boat. 

The first boat that we build had polyester resin over fiberglass cloth.  The reason for this was the cost factor between polyester and epoxy.  At that time epoxy was a lot more expensive.

We found that polyester resin did a very good job of sealing our boat.  We also found that polyester resin was harder to use the epoxy.

The first polyester resin that we used was a lay-up polyester.  This type of polyester resin had no wax and when we finished laying the fiberglass on the boat it remained very tacky.  That made it very hard to sand and even coating again. We then put a coat of finishing resin on the boat, which was hard to do over the sticky resin. This did stop the stick surface but left a wax residue that had to be removed before we could apply paint.

In later projects we found the price of a epoxy had come down quite a bit.  So, we have used epoxy on all our other projects.  We found that epoxy is much easier to use.  It is very strong and sticks stubbornly to wood surfaces.

When looking to purchase epoxy be sure to get marine resin.  Many epoxies are for tabletops and I would not use that in laying up fiberglass on a boat. 

Without a doubt the number one favored epoxy is West Marine System brand.  West Marine not only has great epoxy they also have additives that you can put in to thicken it and make fillers and such like. 

There are other epoxies that you can purchase and Sc110 Marine Epoxy is one of them. It is what we have used on several projects and found it to be very good product.  For larger amounts the supplier ships the epoxy in gallon jugs.  So, if you order one gallon you will get two one-gallon jugs.  One is resin and one is hardener.  I tell you this because neither will be completely full but the combination of the two makes 1 gallon of epoxy.

All epoxy mixes are different.  Some require equal amounts of resin and hardener and some require a lesser amount of hardware to the resin that supplied.  Sc110 is a 1 to 1/2 ratio.  This means that for each one part of resin you would add ½ part of hardener.  Example, one cup of resin would require ½ cup of hardener.

I thought like a lot of men do, that if ½ C was good than a little more would make it harden faster.  But that is not the case.  When you mix epoxy, you must mix exactly as the instructions say.  When I added too much hardener, rather than harden quickly, it didn’t hearten at all. I had a real mess up my hands.  That was when I founder that white vinegar helps to remove some uncured a epoxy.  But the real goal is not to put yourself in a place where you need to remove it.  So be sure to mix you epoxy exactly has the instructions state.

Lastly, I will tell you we found that Polyester Resin does not stick to epoxy very well. However, Epoxy loves Polyester resin. So, our repairs are best done using epoxy resin.

This is the Resin we used on several boat projects with very good results.

On-Board Battery Charger