Steve Firth wrote:
stuart noble wrote:

The catalyst is just that. Any amount will make it go off providing it
is well mixed (the usual bright red colour makes this easier). Shallower
filling goes off more slowly due to less heat being generated.

Not quite true for polyester. The "catalyst" is an organic peroxide that
gets used up as it promotes setting. Use too little and the filler will
never set properly. Use too much and it sets in seconds.

I'm guessing the makers specify more catalyst than is actually
necessary, based on the assumption that it won't be properly mixed. On a
3.5 kg tin, I usually end up with a third of the catalyst unused (not by
design), and I've never had a mix not set fully. Main thing is, you
don't have to be *that* careful with the amounts