How do you you envove onix to steelix in platinum?

Answer 1

let your onix hold a metal coat and if you have a second DS or you don't your friend does , both ofo you go in to the union room in Pokemon center ssecond floor then trade your onix with him then in his ds onix will evolve into stillix then trade it back into your ds again

Answer 2

in order to evolveonix you must lvl it to i think lvl 40

or less