Any good paint over rust paint will work like POR15 or rust bullet. I did this over 10 years ago with a quart of Rustoleum in my used trunk lid. It is still on my car now. If you really want to get the rust out from the inside. Look up rust removal on google with a battery charger and washing soda.
1 Tablespoon of washing soda per gallon of water. (available at Walmart for less than $3.00)
Enough gallons to cover the part in a plastic container.
Piece of junk, clean, sacraficial metal to use as a conductor.
A battery charger. Negative hooked to your part, positive hooked to the junk metal. (could use a battery)
Bubbling action starts and the next day all your rust is gone. (a little longer for extra rusty parts)
ONLY THE RUST COMES OFF. IF YOU HOOK THE + TO YOUR PART, INSTEAD OF -, IT WILL PIT AND BUBBLE INSTEAD OF THE SACRIFICIAL PIECE! The Nasty water and washing soda is safe to now dump into the yard or storm drain. Then you can pour your etch primer right over clean metal instead of rusty.
It really works. I'm still looking for a tub big enough to hold my fender. A trunk lid would be easy to build a box for and line with plastic.