Fred
I have kept proseal in my refrigerator at the hangar for several years & used it for various things "except" anything that has to do with fuel tanks. It seems to cure just fine. I did have a leaking rivet in a fuel tank soon after my first flight. I bought a small package of fresh proseal for that repair. I just felt that it was best not to use the old stuff.
I'm not sure I can answer your question, but if it were me, I wouldn't use it for fuel tanks much after the expiration date. YMMV