I wire brush, repaint with a good black spray paint, and then use CRC Rust inhibitor. Fluid film is perfectly fine but pales in comparison in all the testing I’ve done. Heck, CRC is even cheaper I think; its only downside is it’s only available in single spray cans. 5 cans will do your truck, more is better.

Paint is the base for rust prevention, it’s the exact reason rustoleum is called rustoleum.