BUMPAROO

I know this is an old thread, But i've just been reading a few threads on restoring the underside of a cars.

It seems on most threads i've read most replies recommend different things, some say hammerite, others 2pack paint, others underseal, others a full repaint from bare metal to etch primers etc

I'm looking just to take the surface rust off and protect the underneath of the car.
Can anyone clarify the best way