How did you rustproof your bottom?

You may want to check out "KBS", the coatings does come in many colors and the company has been around a lot longer then Por 15. I went with the blue undercoat and it matched the car prefect, I used about two quarts with a can of their thinner.

https://www.kbs-coatings.com/rustseal-non-porous-colors.html