For rust POR-15 is a very good product
www.frost.co.uk (I think this is the address) sell it, I found it sticks like [PooPoo] to a blanket 8)
Waxoyl as a sprayable light yellow paste is good for inside chassis with the pump gun, and you can buy under-body protection of the thick hard black tar type with waxoyl built it.
Personally, I'd use POR-15 on clean bare rust, waxoyl sprayed into the chassis through any holes etc and then get the Schultz gun and spray the thick under-body protection on with a compressor 8)