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.

What about getting the trailer gavanised after you build it?

Tom