Well of course, it’s always best to remove the rust 1st, if possible. But, to protect hard to reach area’s, One product that I’ve had pretty good results with is “ fluid film”. You can buy it in an aerosol can, or in pint, quart, & gallon containers.
I keep several aerosol cans, and a couple gallon cans in my shop for whenever I need to do touch ups.
The gallon cans I have I pour into a small pressure pot when I’m spraying the bottom side of one of our winter drivers. I’ve had pro good results with it.
I would never apply to the bottom of one of my collector cars, but inside hidden areas, yes.
You can find it at Ace Hardware, on line, or any tractor supply businesses.