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 Equalizer 4pt 10k
2018 Ram 1500 CC 5.7L 4x4 3.92 LSD
2017 Ram 1500 CC 3.6L 4x2 3.55