To complete the chemistry lesson, there is no "hardener" like in epoxy - it is a catalyst. Benzoyl peroxide can be used as a catalyst, but the more common catalyst for fiberglass resin is MEKP.
But the resin crosslink reaction time with catalyst is almost infinitely slow, so an accelerator is necessary to make the reaction cure in any useable timeframe. Cobalt is the most common accelerator used with polyester resin. It is added to the resin by the manufacturer.
If you've worked with many different types/brands of polyester resin, you've probably come across some that fight you, or act unexpectedly - not curing as fast as expected, or kicking off way faster than expected. This is due to the type and amount of accelerator used in that particular resin. It can catch you by surprise. When my resin supply changes, I never start a large layup without first doing some small test pieces to get a feel for the amount of catalyst needed and how the resulting mix tools.
Too much catalyst doesn't stop crosslinking by sponging up radical sites. Well, maybe if added in extremely large amounts where it dilutes out the reaction. Too much catalyst will promote too rapid crosslinking and generate a ton of heat. The rapid crosslinking can trap and lock uncrosslinked resin molecules, and the heat will make the cure brittle and warped, which will be structurally weaker. If it doesn't just catch fire first
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To clarify the epoxy ratio discussion, there are many ratios for epoxy, but each one needs to be exact for its cure. The difference is mostly in the hardener, as the epoxy resin itself is the same for all. So if you have a 3:1 hardener, you need to get that ratio exact. But if you have a 5:1 hardener, it must be mixed 5:1, etc.
That article on polyester mixing ratios is interesting, but has two points that need addressing. They mention an accelerator as optional. It is not, and you won't find many polyester resins without an accelerator. However, one can use additional accelerator for faster cure times with smaller catalyst ratios for particular types of work, and they should have made that clearer. The other point is their statement that polyester resin will not cure without a catalyst. I have a 1/3 gallon of old resin sitting in front of me (to be tossed tomorrow) that proves that wrong. I also have 1/2qt of old gelcoat solid in the can to go too. Neither have seen catalyst. The key is the term "catalyst", which is described in chemical terms as a substance that speeds up a chemical reaction. In other words, that reaction is going to occur with or without a catalyst. Just not in any usable time frame. Polyester resin and gelcoat have less than a year of shelf life until the native reaction renders them solid. Much less if they were stored in a bow locker in equatorial sun (ahem).
Mark
