zinc chromate?

[new2bmw]

just wondering what zinc chromate primer is, and its benefits as opposed to an etch primer. i think the can said it can be used on aluminum and non ferrous metals???? does this mean it can be used on sheet metal panels and underbody etc? what is better the etch or the zinc primer?
thanks guys

[ethree]

Ferrous basically means containing iron which steel (including your body panels) does so I'd just use a good quality etching primer. Ask the guy at the paint shop.