Playing with patina a bit. I picked up a piece of steel as a stand-in for the frame, and started painting one side with red oxide primer. Then I sprayed half of that side in black primer, then shot the whole thing with Willow Green.
Then I started wet sanding with some 600 grit sandpaper. Starting with the red oxide side...
Then the black and red oxide side.
I thought the sanding down to the black layer looked OK, but wasn't wild about the sanding all the way to the red oxide and bare metal.
On the other side, I tried something different.
Let's just assume that the top part of the metal is the top of the tank. I started with some etching primer - a full coat on the bottom, then a light coat at the top. Then I hit it with some gloss black, and finished it off with willow green. The logic was this:
The etching primer will stick to the metal, the black will stick to the primer, and the green will stick to the black. I hit the light areas, it should go right down to bare metal without much issue. Then, once I have bare metal exposed, I can rust it with vinegar and salt water.
So here's where I stand, after a little vinegar treatment. It's not bad, but I won't really know until it's had a chance to rust a little more.
Hopefully I'll get to the frame by the end of the weekend.