Causing surface rust

[YBNORML]

I am building a patina truck. The truck is a 68’ GMC. I have been able to reuse all of the original sheet metal with the exception of the tailgate. A common problem.

I managed to find a very nice GMC gate, I had it sandblasted and I am going to attempt to replicate some patina paint with spray bombs.

The biggest issue I have is that the box tops have been worn down to bare metal over the years of work on the farm. The tailgate really need to somewhat match or it will look funny. I have the original gate but it is waaay too beat up to use.

I’ve heard of guys using acid to cause flash rust, anyone have any info in this, or any other ideas?