When you find the man's missing granddaughter on the S.S. Aqua then entertain her by playing hide and seek the man will give you the metal coat. You can only do this after beating the elite 4.
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Onix > trade w/ Metal Coat > Steelix
the Pokemon that evolve with metal coat are steelix and scisor
go to the ship and find the granddaughter and old man gives metal coat. Also available at the pokeathalon
Check daily at the pokeathlon dome. On certain days they will offer to give you a Metal Coat in exchange for points.
if you cach an onix there is a posibility that it could be holding a metal coat