In article ,
"For example: John Smith" wrote:

what can I do to the inside of square or rectangular steel tubing so that
over time it will not rust from the inside out? I am thinking of making a
6x10 utility trailer with a boxed frame for extra strength and I live on the
Gulf Coast of Central Florida where the air is salty and humid and I plan on
having the trailer for a long time. The process would have to be done before
welding so whatever the process is, it must stand up to the heat from
welding. Thanks in advance.

If I recall correctly, with aircraft stuff... engine mounts, fuselage
and other welded up stuff made out of tubing, they drill a couple of
holes, and partially fill the tubing with warm Linseed oil. Rotate and
slosh to get complete coverage and allow to gravity drain for a while,
then plug the holes with drive screws.

At least I think it was Linseed oil...

A lot of that stuff has been flying for decades now, even off of salt
water.

Erik