Years back I bought a bunch of ammo cans that had some funky chemical all over them, including the "rubber" seal gasket. Here's what I did & 15 years later they're still fine.
Wipe with denatured alcohol, or yellow "heet" (same thing), till any black surface crap comes off on the cloth. This gives you a fresh surface to work with.
Buy some "armorall" wipes & use them to run a couple of coats of the chemical into & all over the rings. Don't worry if a bit more black stuff comes off,as long as it's not big chunks. Repeat 2~3 times waiting till the last coat soaks in before applying the next.
After the last one has "dried" close 'em up & don't worry for the next 30+ years.
Allan Quatermain: “Automatic rifles. Who in God's name has automatic rifles”?
Elderly Hunter: “That's dashed unsporting. Probably Belgium.”