Danesh, I'm glad you raised this issue. Because the frame is mostly hidden, it is easy to believe all is well. I learnt the truth when I went to an RV wreck yard,and saw several late model Airstreams which had been torn apart. Fifteen year old Airstreams had such severe corrosion in the area near the rear bumper that they were falling apart. I removed my rear bumper by drilling out the 4 corroded set screws (later replaced with stainless steel set screws with zinc chromate paste to prevent electrolytic corrosion) . When the bumper was off I chipped off all the loose flakes, treated the areas with phosphoric acid, and then used Rustoleum grey paint. You also then have access to the open end of the 25 foot long frame box sections. I made up a 25 foot tube, pushed it right down each frame section, connected this to a spray gun and compressor, and pumped rust-proofing fluid down the tube. I had a radial spray fitting on the end, and packed it so it was at half height in the box-section, and gradually withdrew the tube, aiming to spray the whole inside of the tubes. I bought the JC Whitney car-body rust-proofing kit, but my 2 HP air compressor was insufficient to spray the thick fluid down such a long tube. It just dribbled out the end. I am going to find a professional rust-proofing outfit to do the box sections with a more powerful setup. I am also going to remove the lower curved body sections, one at a time, and treat the outside of the frame.
It's a time-consuming task, but I aim to hand my trailer down the generations, preferably without having to take the body off the frame for a re-build. Good luck with the project. Nick.
Nick Crowhurst, Excella 25 1988, Dodge Ram 2500 Cummins Diesel. England in summer, USA in winter.
"The price of freedom is eternal maintenance."