Why Do Some Mint Sets Go Cloudy?

First of all, I killed two hours trying to use the numismatic search engine to get an answer to this question and, while I learned some things I didn't know before, I couldn't find an answer to this question.

In the good old days before there were 28 coins in a mint set, you'd get two cellos with 5 or 6 coins with the major coins. One had the blue stripe the other a red, for Philly and Denver issues. I have multiples of the older mint sets (60s, 70s & a few early 80s) My problem is that as I go back through these sets, either one side or the other has coins with a powder-like coating. When I've asked people about this in the past, most suggestions start with 'try an acetone bath' and then go in all directions.

Assuming they're not subjected to radical variations in temp, light, and humidity a good set of uncirculated coins for a year should have coins similar to those that left the mint, My problem coins appear to be coated with something. All coins look a lot like coins that have been intentionally given a matte surface, such as the "matte nickels" in two mint promos back in the '90s.

For a give year I'll have one envelope with nothing wrong with it. the next envelope (same year, same source) will have foggy Denver side. I think you get the gist. Unlike toning, I find these issues unattractive and would like to keep them as uncirculated, but bring back some of the "brilliant" that dealers tout for fresh from the roll (or mint set) coins.

I don't think pictures would help with this very much. Instead, imagine this visual. You have two bright nickels on a hot, dry day. you put them on a bench and blow fine dust upon them. Then pick up one nickel, wipe the dust off with a handkerchief and place it back next to the dusty one. That's what I see in some cello sealed sides of a mint set. Anybody care to offer some wisdom on 1) cause, 2) selectivity, and how would you go about conserving the scaly coins?

I'll be in and out today so I'm looking forward to your insights when I get back to my computer.