Re: Etch ?
A surface can be etched for painting or zinc galvanizing by sandblasting it down to the bare metal, then prime and paint. There is a zinc "cold galvanizing paint" that can be bought and applied, but it's probably not near as good as real dip-tank zinc galvanizing. No need to spend more time and money on a trailer than it's worth, especially if it's all eat up with rust and corrosion. Sometimes the worst corrosion is on the inside of the channels, tubes, rails, and you can't even get to it with paint.