How can you tell if oil based polyurethane has gone bad?

Answer 1

I've been a woodworker for 35 years, and the only time that I've seen it "go bad," is when it hardens like a rock. Otherwise, I've always used it. As far as I know, there is no expiration date, nor any reason why it would be considered unusable. I've often thinned oil-based polyurethane, not because it's bad, but because it's too thick for my intended use.

Answer 2

If it was opened more than a year ago, test it on a piece of scrap wood, if it is still good it will harden according to the directions on the can. If it has gone bad it will stay tacky for days. Always write the date that you first opened a can of finish on the can so that you can tell how long it has been since you broke the seal and started the degradation process.

Answer 3

It loses it's fluid medium and feels more like resin.

As it ages further, it gets thicker and thicker, eventually becoming solid in a deep amber color.