Make sure you give it a really good scrub in degreaser and wash it off well, when dry use T wash and then use a good chassis coating system. Specifying one will cause some argument so you takes your pick of paint and primer. However up to that point is a given.
On my aged but sound galv chassis I used in several coats a whole can of rustoleum special metals primer for galvanised and non ferrous metals, it did seem a bit chalky, i dont expect to use that brand again, some research came up with phosperic acid which i regularly use as rust killer/ neutraliser as being a good mordant to use on galv,
i overpainted it with this from my local farm supplys
And then a good anti corrosion tractor enamel,
I also sprayed copious waxoyl into the insides,