i think its the coins condition themselves sometimes.. as in what kind of crud is on them. our red clay dirty ones are tough to clean. zincs often are swiss cheese rotten. gravel and ammonia work best but get them out and rinse. i always seem to miss a zinc in the clad but they still fit a coin roll and the bank takes them. i tell my teller i know--These need to go to the mint! holey zincs.. my tax money made them, and they fit in the rolls just fine. good coins i mark for her to use. need to know your regular teller.. sweet thangs they are! no fees there. i have tried clr, vinegar and salt, lemon juice, etc... but ammonia seems to work the best with gravel. i mixed something one time and it popped the lid off my tumbler barrel lol. i just get them clean enough to roll.
i try all the things you guys say on common clad... aint gonna hurt nuttin....