Does polyester resin or catalyst hardener deteriorate with age? I've got some old stuff that didn't seem to set as I expected last time I used it, but it wasn't in a critical application so I covered it with filler and forgot it. I need to do a repair that matters so yesterday I did a trial mix of about 2 parts resin to 1 part hardener. !8 hours later (in a cold garage overnight) it is still not hard, but has reached a rubbery cheesy state. Should I throw it away and buy new?

Are you sure you're using the right catalyst?

It may of expired or the temperature in your garage was too cold. Resins need a certain ambient temperature to cure. .

Try bringing it inside and see if cures properly.

Is it definately polyester resin? If so your not using the pink hardener in a tube are you?

Very unlikely to be too cold to go off in that time, even in Derby!

And your mix is completely OTT. Polyester typically uses a ratio 0f 100:2 so 2:1 will in itself not exacly be ideal...!

But yes, the resin can take on moisture, and the catalyst degrades over time... try a mix at the correct ratio (assuming you are referring to a polyester and catalyst, and not an epoxy and hardener - they are not interchangeable!). Most hand laminating polyesters will go off in the pot after no more than an hour at a 2% mix. If you spread them thinly, they will take longer, and if you completely alter the ratio, that will also have an effect (you can increase the catalyst ratio to about 10%, but you should get a very fast reaction, and greater exotherm and shrinkage - best to stick to the specified ratio).

In my garage in the cold a few weeks ago i mixed up the catalyst with my polyester resin about twice the strength in +3 degree heat and it dried in 3-4 hours....

I guess your using the wrong catalyst. i dont think it expires i have left it open for 3 or 4 months in the past and it just hardens on the top like an ICE layer....

In my experience old Resin and Hardner (Polyester) usually hardens faster as it ages - not slower.

I suspect your mixing ratio has caused the problem

I'm using the right stuff - Polyfibre resin and catalyst bought together from a specialist. As I said, its old but both look to be in perfect condition.
I've just mixed up another small batch using more sensible proportions, so I'll let you know what happens.

Too much hardener, will make it a rubbery, try the correct ratio and it should be fine.

It could be age but it could also be temperature.

My hairdryer was never the same after being used to warm up some lamination but it did the trick.

Almost certainly too much hardener, that is waaaaaay too much, and will stop it curing properly.

I have some resin that is 6 years old and went down to minus 16 deg c last winter and it still cures fine.

I'm with JAG. I find that old resin sets very quickly compared to fresh stuff.

Thanks for advice guys. Batch 2 mixed with proper proportions has set properly.