Does Sanding Sealer Go Off?

I have a can of sanding sealer (Feast Watson) that I bought about 3-4 years ago, how long does it last?.

Hi Sam . If you dont seal the can 100% it goes off. It can be a total loss of 4LT. And the more air that gets in the can as you use it and the level drops sets a skin on top. Ive never seen a full unopened can go off but they only sit on the shelf with me for 6 months maybe before they get used if I have a few 4LT cans.

Thanks Rob, Its still three quarters full, no skin formed but upon opening looked like the medium had separated from the thicker grey stuff. I just mixed up again and it all seemed fine.