Heres a crazy idea..............You say there isnt any body filler on the outside of body? Its raw metal?
Why not just DA the outside with 150 grit , and clear it to protect the steel from rusting ?
It would be a neat look instead of painting it with a brush.

or another idea, you could try spraying the body with red oxide spray bomb (several coats), then get a color of your choice and spray it with a few light coats and use a red scotch bright pad and rub lightly untill you start going through the color to the sealer, it would look like old time paint fade , then clear it with a slightly flatened clear to protect the look. The simi flatened clear would give it that old wear look. Because a fadded paint job has no shine left.

SprayTech