painting on metal help !!!!

query on painting on metal letter box

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i bought a metal letter box which i put up outside my house,

im nearly sure it is a galvanised steel but i could be wrong,

after about two months it went a very brown tarnished colour

so i decide to paint black hammerite on it but to be ho0nest it never took to it and was very difficult to put on was like it had not been painted in ages and had bad brush marks

so i decided to paint it white but that was worse what have i done wrong and how can i rectify the problem please sorry for bene a nusiance any help would be grateful at this stage