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I know that less folks use varnish as some other available materials, but i must ask anyway.

Does varnish have a shelf life? I know shellac is around 6 months, but I'm not sure about varnish or anything else. In case it matters, I have some tung oil varnish that I purchased around 1 year ago and just let it cure for 3 weeks after finishing a mandolin with it and the finish started peeling after I began wet sanding.

I think one or more of several things could have happened:
1) The varnish did not properly adhere to the shellac sealer coats. (definitely happened)
2) The wet sanding paper had something in/on it which dug into the finish and pulled a bit of it off.
3) any other issue I can't think of right now.

So, since the finish is ruined I decided to see how tough this varnish is and man, it took some effort to get my fingernail to scratch it!

Has this happened to anyone else? How can i make sure it doesn't happene again?

By the way, Merry Christmas everyone! (well, christmas eve)

Geoff,
What was the brand and name of the varnish you had the problem with? We need to start there to figure out what went wrong.
As for shelf life: it varies on the varnish. Most oil varnishes will skim over in a partially filled can unless you use Bloxygen or something similar. If the varnish skims over, you are pretty much out of luck unless you strain the remaining varnish through a very fine mesh filter. Even then, you are spreading partially polymerized varnish and it may not flow out very well or dry completely even.
As an aside, I was speaking with Paul Fischer last fall about the varnish he uses. He managed to lay in a stock of Behlen's 4 Hour varnish before it was discontinued. He says it improves with age and will not hesitate to use 4 Hour varnish that is decades old. Interestingly, he said it does not skim over in a partially filled can. That leads me to believe that there was some sort of spirit varnish component to the 4 Hour in addition to a polymerizing oil varnish. Odd that Behlen ceased production as it was a favorite of many.

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