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Making a decison on "old" varnish

Bill Tindall, E.Tn.
Varnish is a mix of solvent, resin, crosslinking oil and catalyst (and delusterant ). What could go wrong in time? What would wrong look like?

Solvent could evaporate. Varnish would be obviously more viscous. Remedied by adding more solvent. Never will happen in a metal can.

Resin- absent oxygen or light unlikely any chemical change.
Oil- again, absent oxygen or light unlikely any chemical change.
catalyst- stable for ever
air/oxygen leak- varnish will cure and separate from solvent as some sort of suspended solid gunk.
Delusterant settled- might not be able to re-suspend. Fatal.

But if there is any lingering doubt, apply some to a test piece. If it hardens in an expected amount of time its as good as new.

I have no reservations using "old" varnish because it tells me if it is still good either by not showing separation or by a test application. The best reason to use old varnish is the proclivity of companies to reformulate. The new stuff my not perform like the old stuff. Example, I have never found a replacement for Sherwin Williams "Beautylock". I nursed a gallon for at least 25 years before I ran out.

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