The data sheet that came with my WLS primer says it has a shelf life of 12 months minimum at 70° F.  There is no maximum shelf life published.  If it is like other epoxies, I expect the shelf life is much longer than 12 months, particularly if it's not exposed to extreme temperatures. 

 

You can always do a test sample.  Mix up a small amount and apply it to a scrap piece of composite that is prepped in your customary way.  Give it a week or so to fully cure and then try scratching and sanding it to see if it acts the same as it did when fresh. 

 

You can also do an adhesion test:  cut an "X" on your cured sample, with each leg of the "X" an inch long.  The cuts should cut completely through the cured primer finish.  Cover the "X" with tape that's aggressively sticky (Scotch tape works).   Make sure the tape is stuck as tight as it will stick and then peel the tape off.  None (or very little) of the primer should lift off with the tape.  If it does come off, it means that either the primer has gone bad or your customary prep methods need to be improved.

 

I've had a couple of good telephone conversations with WLS' paint chemist, Victor.  He's always called me back and seems to know just about everything there is to know about aircraft paint.  Maybe he was on vacation when you called; I'd give him another call. 

 

Best,

Dennis Johnson

Legacy #257 (in final WLS primer)