I have a project that I want to do this weekend that involves making some fiberglass parts. I have about 3/4 of a gallon of resin left from a small project I did about six years ago. The resin still sounds liquid when I shake it but I don't want the parts to fail because of old resin. Should I just get new?
A new gallon is about $30.
What's the part (and your time to remake it) worth to you?
In reply to SVreX:
Okay, lets say I decide to buy new. What would be the appropriate way to dispose of the old resin?
6 year old resin is unusable. I tried using resin around that age that was still sealed, it took several days to set and when it did the part was brittle and snapped easily. I ended up having to pretty much start over. 
Nick (Not-Stig) Comstock wrote: In reply to SVreX: Okay, lets say I decide to buy new. What would be the appropriate way to dispose of the old resin?
mix in some hardner into it and pour into a container of some sort and let harden.. then throw in trash. Just make sure it does not get exothermic
In reply to mad_machine:
It seems like your brain is still functioning while mine was shut off hours ago 
Ive used resin well over a decade, and it worked fine.
The hardeners on the other hand, those can go bad.
Do a small test piece. Then you'll know for sure.
I've used resin over 2 decades after I opened it. Probably pushing 3 decades. Worked fine.
HMM, conflicting stories...
I've had both experiences. I used 10 year old stuff many years ago and it worked fine. A couple years ago I used some that was less than a year old and it wouldn't harden, had to start over with new resin & hardener. Now I don't even try stuff more than a couple months old, not worth the time, just start with fresh product. Perhaps slightly different chemical formulations are used now? The activator seems to evaporate through the plastic tubes it comes in pretty quick compared with the stuff from many years ago.
Nick (Not-Stig) Comstock wrote: HMM, conflicting stories...
...and therefore, is it worth $30 to you??
You're gonna get different answers because your question is unanswerable.
No one know how tightly sealed the container was, whether it was stored in a hot space or conditioned, what the formula was, who the manufacturer was, etc. etc.
Folks are giving answers, but they are not good ones.
Spend $30 if it matters. If it does not, do an experiment, and report back to us. 
foxtrapper wrote: Ive used resin well over a decade, and it worked fine. The hardeners on the other hand, those can go bad.
Kind of like paint. It's lasts for a long time if sealed. The hardner on the other hand is only good for a couple weeks. Or at least the PPG I was using did.
What kind of "resin"? Epoxy? Polyester? Other? Some last better than others. 2 part marine epoxy resin has a loooooong shelf life, even when open.
Based on what some of the posts have said I would guess that most of the problems using old resin could be the use of old hardener and not the resin itself.
mad_machine wrote:Nick (Not-Stig) Comstock wrote: In reply to SVreX: Okay, lets say I decide to buy new. What would be the appropriate way to dispose of the old resin?mix in some hardner into it and pour into a container of some sort and let harden.. then throw in trash. Just make sure it does not get exothermic
And if it actually hardened properly, then you wasted what you had.
If this is a hypercritical application, buy new materials. Else mix a test batch and decide for yourself.
It's not a super critical piece. I'm going to make some wind deflectors that mount to the side of my fairing to block the wind and rain from my hands for the winter.
I'm going to try what I have with new hardener. Worst case it doesn't work and I have to buy new.
I have had old catalyst be an issue, but not the resin.
TEST PIECE, DUDE!
In reply to Appleseed:
If I'm going to go through the trouble of mixing some up I'm going to make parts. But it's going to have to wait until I get my next allowance, I blew most of this one on saddlebags.
If its polyester resin, it does have a shelf life. Epoxy resin is more tolerant. I have some 8-year old West Systems epoxy and the directions say that it is basically good as long as it is still liquid and still gets a full, non-tacky set, but after X years you should do a test piece first.
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