Agree with 440, If you can blast it, do; if not wire wheel, cup brush, knotted on a grinder if you gots.
Start with simple green or purple power to degrease too. Takes time but you'll be far less frustrated and much happier with the finished product.
I would then hand brush this stuff on. works well, not as good as POR15 with regards to self levelling, but much less expensive:
It will leave brush marks to some extent, but I love it in hidden areas. You will need to prime it within 24-48 hours. Rust Reformer from rustoleum in spray bombs works and doesn't necessarily need a top coat. The side sections were black from the factory in many FSJs anyways.
Just my 2 cents. I have an 18 yr old amateur resto done this way and the rust did not come back. Others will weigh in.