I painted my lines the first week of ownership. They looked just like your pic, though I saw worse on other trailers on the lot, especially ones built in the winter. Must have been towed down when salts were on the road up there.
Before spray painting, I used some emory cloth and fine sand paper to knock down the surface rust, then cleaned with some paint thinner and microfiber towel.
I used the higher quality industrial black spray paint from Home Depot.
I also hit the little bits of surface rust around the tongue, frame and axle hardware.
2015 Wildwood 26TBSS / Fastway e2 1k WDH
2016 Ram 2500 CC 6.4L 4x4