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Changing color of rust finish

I made for a customer a iron gate. She wanted it to be rusted. I built the gate and hired a guy to sandblast the scale from the steel to help it to rust form quickly and evenly. Unfortunately his equipment broke down and was unable to finish the job and I could not find someone else to finish it. For the final sections that were not sandblasted I chose to help the rusting process along by spraying it down with water and muriatic acid ⇦ this on eBay or Amazon [affil links] . The problem is that now she has a two tone rusted gate. The sandblasted part a dark brown and the acid treated part is orange. After the gate has been in place for nearly a year It is clear that the orange colored rust is taking over. The customer wants the brown colored rust. Besides sandblasting the whole thing. Is there a way to fix this?

Charles Masters
Ornamental metalwork - Yelm, Washington, USA
2004

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February 17, 2011

Well, the customer did get a rusty gate - she just doesn't like it...

However, my bet is that you have two different oxidation states and that you can resolve this by heating the orange rust - not too hot, maybe 400 degrees F. - just to where oil would smoke or a bit more. I'd use a weed-burner and a 20# tank of propane... Good luck.

Rust reformer - phosphoric acid - that'd do about the same thing, but it's costly and if it doesn't work, if she doesn't like it, you'll have to sandblast...

A. Benway
rancher - chico california