Materials: Resin and Fiberglass
For our purposes there are essentially two kinds of resins to consider: Epoxy and Polyester. Both are two-part compounds. Epoxy consists of a Base (part A) and a Hardener (Part B). The components that make up Polyester is usually referred to as Resin and Catalyst.
You should be looking for Lay-up Epoxy. It has a very light consistency compared with Epoxy glue that most are familiar with. The light viscosity is essential to wet the fiberglass.
If Epoxy is mixed in incorrect proportions, it can spell disaster. Too much Hardener and the resulting laminate is weak and brittle. Not enough and it won’t cure. Instead it will stay tacky for weeks and never develop any sort of hardness.
Controlling open-time (time before it cures) and pot-life (effective time to work with the resin) with Epoxy can be tricky. Many manufacturers offer several options of Hardeners that gives different open-times. Temperature plays a large role – lower ambient temperature leads to longer cure times.
Often a Hardener that provides a long open-time will cure tacky in humid weather. So that is one other thing to consider.
Polyester in that regard is much more forgiving. The catalyst is just there to accelerate the crystallization that is technically already taking place, only really slowly.
So the speed at which polyester cures can be controlled by the amount of catalyst added.
Polyester resin has a much shorter shelf-life. I seem to recall you should use it within a year (I could be wrong, check with manufacturers).
Here is a short comparison between the two:
• Non-toxic when ingested or fumes inhaled – can ship by air.
• Little or no odor
• Critical mixture ratio for optimal strength and proper cure
• Almost infinite shelf life
• Difficult to control pot-life and open-time
• Repeated contact with the hardener will lead to sensitization
• Will not dissolve thermoplastics such as polystyrene foam
• Volatile fumes – can only ship ground
• Strong and nauseating smell
• Relatively inexpensive
• Mixture ratio non-critical
• One year shelf life
• Change mix ratio between Resin and Catalyst for infinitely variable open-time
• No long-term health effects as long as a proper respirator is used
• Will dissolve thermoplastics and most foams
Which to choose? Well, if you can only work in your house or in an attached garage or basement: it’s Epoxy. The fumes from the Polyester Catalyst is so permeating that there is no way to work with it under those circumstances. Even your clothes will stink.
I wouldn’t recommend working outdoors, but it’s an option where Polyester may work out.
My shop is in the basement, so I'm strictly using Epoxy.