Damage will be pitting but varys greatly in extend, you said dusting and most spots came out with rem oil so doesn't sound to bad. The spots that didn't come take your rem oil and some 0000 steel wool- work in small cirlces the effected area and wipe clean. Effect for the future means unprotected portions of the guns (finish removed - bare steel). Solution always keep well oiled with decent light gun oil both for field and storage. When I re-appy during storage I wipe down the rifle with a clean cloth andthen re-apply the gun oil.

Storage you may want to look for dehumidifer for the the room in which the guns are kept. By any chance where they in a gun case? If so that can be a source of the troubles, you may consider putting them in a closet out of the case then installing a lock for safety. I store all mine in locked gun cabinets in the basement, if though I don't have humidity issues in my basement I have rods in the cabinets just in case. I oil while in storage as a precaution with Birch Casey's Sheath.