Sheet metal building...How to..??

[Doc Holliday]

Although not quite the same, I have built a couple of kits with the Campbell's corregated tin siding. Like you, I didn't want a shiny new building so I weathered it using a series of paint washes starting with shades of gray and then several different rust colors. A started with a spray of Dullcoat to give the paint something to stick to. Be sure to pay attention to the individual pieces of siding as they always weather differently. Here's a pic om my Idaho Springs Mine. Post some pics of yours when done.
Doc