Polyester resin curing oddness

[Keith 66]

Using polyester resin there is a limit to the amount of catalyst you can use, they say 1 to 2% & though you can use more it produces an weaker cure. At this time of the year you are pushing the boundaries of what is possible without heating, Infra red is most efficient.
To get a fast cure in winter without heating you can boost the resin prior to catalysing it with Cobalt accelerator, it must be mixed in well before you add the catalyst & you dont need much. Adding cobalt means that you can use polyester down to zero degrees or even below successfully without heating.
Roofing companies routinely use cobalt accelerator for working outside.
It is not that easy to get hold of today as if it comes into contact directly with Peroxide a nasty reaction often explosive occurs.

 

[TernVI]

I think most 'polyester layup resin' is 'pre-accelerated', already got some accelerator in it.
IMHO best not to mess with adding more without reading the manufacturer's gumph for the exact resin you have.
Some resins are suitable for colder weather.
A good start can be to store the resin in a warm room the night before or pour what you are going to use into a pot and stand the pot in a bowl of warm water.
Don't over do it!
The resin I have is good down to 15degC. It's not hard to achieve 15degC. Around here, if you can't get 15degC, you are going to have damp problems anyway.
Measure the catalyst, I use a little throw-away polythene pipette.
Get an IR thermometer, check the temperature of the surface you are trying to lay up on.

I once had a grp job go really badly sticky, I think it might have been old catalyst. Or it could have been the resin, but that usually works unless it won't come out of the can.
Other times I've seen it go wrong, it's just bad guessing of catalyst. Or maybe failure to stir the mix?
Polyester is cheap. Buy some fresh stuff of a respectable brand and avoid wasting cloth etc.