How good does Rustoleum work over rust ?

mostly asking about spraying it on the underside ,
the frame rails , underside of the floors and inside of wheelwells ,

the metal is not rusted thru , just has some light surface rust , I will knock down the flacky stuff,

do I need to rough up the surface or does Rustoleum have some "bite" when its put on ?

I just think that covering the bottom will make the car look better and hopefully keep it from rusting further....

I planned on using a dark grey primer color Rustoleum.

Beaulieu

In Southern California