I bought a used model 2, with a non poweder coated frame. The lower tubes below the seat and further back were rusting next to the fabric. It appeared the fabric was rubbing off the paint, and lots of dirt had accumulated on the lower fabic (between the fabric and tubes), got wet or was holding moisture and rusting the exposed tubes. Not to bad though, but it would have continuted to get worse if I didn't addess it with fresh paint and a cleaning. Maybe regular paint is not tough enough, I really don't know for sure, but keeping the inside of your plane clean and dry will make a world of difference in the even the metal get exposed.

Roger