Thank you! I paint mine with acrylics, so I prefer to seal them just to be sure, though they're hardly items you'd take out of the display case and play with. I used regular polymer sealant a while ago, but I feared, since it's not acid-free, that it would yellow over the years as with some paintings I've helped frame in the past.
I've been experimenting with Mod Podge Hard Coat and am getting great results for what I need, so I think I've found my solution. I still don't know why this and my hieroglyph piece crackled, but that little expensive bottle is in the trash now, so it won't happen again.