Just four years ago all the coins in '92 mint sets looked OK except for an occasional tarnished coin that was easily cleaned in acetone and a carbon spotted penny here and there. I was able to see a bunch of sets again recently and was surprised to see 25% of the '92-D pennies are developing carbon spots around the periphery. Fortunately this date is relatively common in Gem so loosing such a large percentage isn't a major disaster. But I am concerned that if they are going so fast they could all be gone in very short order like the '68-P cent. Collectors need to get all their coins out of mint set plastic and not just the 1965 to 1981 PVC plastic. Even the later sets are going to prove unstable over time. Ideally the coins should be soaked before long term storage but I believe proper long term storage is more important than removing residues.
