Berger Lacoloid Paint is a very good way for anti-rust coating. As Lacoloid is a marine paint and is used to paint the boats and other marine equipment that are in constant contact with water. After painting the car under-body with lacoloid, you will be tension free about rust.
Before painting the car make sure that there is not even a bit of rust left. This may cause the metal to corrode. BTW
I use simple Berger Red Oxide Primer for coating my car under-body.