One way to go about it is to play through Diamond, Pearl, or Platinum, beating the Elite Four and obtaining the National Dex, then going to Iron Island. You then go into the house, talk to Byron, and he'll give you the Metal Coat. Then, you can call a friend over to borrow their DS. Give one of your Pokemon the Metal Coat to hold, then trade it over to your HeartGold via Union Room. That's one way, though I believe there's another involving going through the S.S. Anne in Kanto and getting it from the captain.
Good luck to you in getting it.
pokeathlon
Make it hold a Metal Coat and trade it. I recommend you to ask someone to trade Scyther with a Metal Coat to you.
You can't make the metal coat, however, you can obtain it as a drop from a boss pokemon.
the Pokemon that evolve with metal coat are steelix and scisor
he evolves with a metal coat when you trade him
Someone has to trade you an Onix while it's holding a Metal Coat.
Make it hold a Metal Coat and trade it. I recommend you to ask someone to trade Scyther with a Metal Coat to you.
You can't make the metal coat, however, you can obtain it as a drop from a boss pokemon.
In Pokemon Liquid Crystal, you can obtain a Metal Coat by talking to the man inside the Pokemon center in Olivine City. He will give it to you as a gift.
You need to give it metal coat and trade to evolve it to Steelix.
By trading it while it's holding a metal coat. You can get a metal coat as a prize at the pokeathalon or you get it from some guy, but i can't remember who.
Get an onix have it hold the item metal coat then trade it to someone then take it back it will be a steelix now.You catch an onix then trade it while it's holding metal coat.
You must Trade Onix while it is holding a Metal Coat in order to Evolve into a Steelix.
the Pokemon that evolve with metal coat are steelix and scisor
You have to give it a metal coat and trade it.
he evolves with a metal coat when you trade him
The Metal Coat can be found on Iron Island.
he does not evolve at a certain level he evolves into steelix with a metal coat when you trade him. yourwelcome to whomever asked the question.good luck on Pokemon heartgold.