If a manufacturer sells a trailer at wholesale for $3,000 or so, would it hurt them to spend an extra dollar or two on paint and actually paint the whole frame? Are they that hard up for money? I would never buy a new trailer from these guys after rebuilding one from the ground up.
I bought a used trailer that I knew had rust issues. After tearing the whole trailer apart I figured out they only painted the parts of the frame the customer could see. The parts of the frame inside the wall are not painted and have rust on them too.
I suppose when you manufacture 10,000 trailers a year a penny here and a dollar there adds up, but if you want a customer down the road you can't do things too cheap.