Bob,

Since PVC tends to gel once it has been opened, I prefer to buy the
smallest cans I can find. That way I can use what I need on that
project and throw the remainder of the can away. Then I'll have a
fresh new can for the next project.

Would storing the cans upside down make them last longer?

I'm no chemist, but I wouldn't think it would make any difference. Once the
can is opened, the cement has already started reacting with the air. When
you put the lid back on, the air is trapped in the can regardless of which
orientation you put it in.

In my experience, the entire can turns to jelly evenly. It's not like it
forms a skin on top or anything.

Of course, it would be any easy experiment to open a couple of small cans
for 15-30 minutes, dipping the wand in and out several times to stir it up
as you normally would. Then seal them up again, storing one upright and the
other upside down. Wait a couple months then check them both to see if
there's any difference. Maybe you'll prove me wrong.

Anthony Watson
www.watsondiy.com
www.mountainsoftware.com