I'm going to use that stuff under the wheel wells.
Not much rust at all on this bus but since I have a gallon of it, I'm going to use Rusty Metal Primer from Rustoleum for a lot of the upcoming paint prep.
For the battery compartment I used Rust Converter then Rust Metal Primer, then topped it off with the Appliance Epoxy. The back bottom corners behind the batteries were the rustiest part of the bus and they were still solid. Surface rust only.