Well out of curiosity I went and dug out a couple of minis I painted with the vallejo alcohol base metallics (vabm for short from here on out heh) a couple of years ago just to check to see if mine had done something like that and they\'re fine like the day I applied them.
Considering the minis, like myself, do not see any sort of sunlight or rough handling they\'ve, as expected, held up rather well. Maybe 4 or 5 years old by now I\'d reckon. No special treatment or sealants used. Just applied like any other metallic paint at the time. From the bottle, onto the mini.
To be fair, the two colors in question are not Bronze as per the paint you noticed the bad stuff happening to but Silver and Gold. Could be an issue specific with that color, couldn\'t say if that be the case as I\'ve not used the bronze variety.