hey guys, im a very experienced welder and am comfortable with my abilities, but am not sure if im going about this in the right way. im planning a trailer build thats going to be as perfect as possible with a very clean look. its just going to be a 6x12, but i plan on using tubing for the perimeter frame. with 45's at each corner, i hope to be able to seal out any water, salt and such. all i want is to be able to rust proof the inside of the frame, but am not sure how to do it. i had a thought that rust needs oxygen to form right? so if i weld a threaded bung and added a guage, and added a slight vaccuum to the inside of the frame, then not only would i beable to tell if theres any pinhomes in the welds, but it would also rid the insides of oxygen which in turn would hopefully keep the insides from rusting out?

im very set on using rectangular steel tubing for the main frame, but i dont want them to fill with water and such and rust out from the inside out. i should add that i live in minnesota, so rust is everywhere on everything that drives on these roads do too higbh salt usage.

any suggestions are welcome!