How do you keep your polyurethane from spoiling?

[Guses]

For those of you who work with wood, how do you keep your polyurethane/varnish from spoiling?

It is much much more economical to buy varnish or PU in the larger containers. Where I live, the 237 mL jar is 10$, the 936 mL size is 20$ and the 3.97 L huge container is 50$.

Since I got into woodworking and doing my own furniture, I figured I would purchase the  middle size because of economies of scale. I even considered the largest size but decided against due to size, risk of spoiling and diminishing return.

I was doing some shelves recently and wanted to give them a few nice PU coats after staining. I found that my polyurethane was all shriveled and dried when I opened the container last night. There was about one third of the container left...

I am pretty sure the container was shut tight (obviously it was not airtight though). For reference, the container was about 2 years old at this point in time.

All that to ask how do you keep your poly from spoiling? Do you buy larger containers? Does temperature matter?

Can you "disolve" dried poly to reuse it? I have acetone and terebenthine readily at my place.

[Woodshark]

Air in the container is the problem. The simple thing is to add marbles to the can to raise the liquid up to close to the rim.  Still, it does not last forever. As it gets old, it loses it's ability to cure. Two years is about it. Also, avoid freezing it as that is bad.

Something I've thought of but never tried was pouring the unused poly into a empty water bottle and then squeeze out the air and cap off. I've done this with house paint (for touch ups) but have not tried poly. I fear it might, just might slowly disolve the bottle.

[Guses]

Now that you mention it, I think that I could probably get one of those metal cans that are thin and long and squeeze it so that there is no air.

Would flushing with CO2 before closing the canister help?

[lakemom]

Put a layer of plastic wrap right on the top of the liquid in the can.  Put another layer of wrap over the top of the can.  Tap the lid on tightly with a mallet.  This should help it keep for a while but with most paints/varnishes about 3 years is the max you are going to get out of it.