Protection question

[MSLD]

I am fabing some new rockers out of boxed steel and will be needing to tie this project into a floor replacement (I live in Ontario where the roads are salted). I will be cutting out a lot of rust and plan on doing everything in my power to prevent this repair from becoming a once every two year type of affair. My questions are...
Has anyone used asphault undercoating in addition to oiling yearly?
How does it hold up? (the inside will be box lined)
and does anyone have any better recommendations?

I appreciate the input
Matt