I was told several years ago, by different people in the hobby not to use anything on the outside of the glass... whether sprayed or otherwise (that things may overtime seep into the tank) and never questioned it because if my father saw me using one of his beer mugs for milk, he was pissed. Never questioned that either, just didn't see the point in risking chemicals in my tank, or pissing off Pops over one of his mugs.
I have since, after seeing responses here, researched it myself and have come to the conclusion that the only way transfer into the tank would happen would be by human error.
As for the beer/milk thing your guess is as good as mine.. didn't research that because I drink my beer straight out of the bottle.
Glad I was able to give you a chuckle at my expense :flirt:...and just for the record... the tank isn't leaking :tongueani