I've always used LPS 3, Which as with most products it doesn't always cover every aspect of metals protection for 1 it is nasty messy stuff But recently a friend and partner has turned me on to a product he sells. Made by KBS Products, called cavity coater
It has a 4000 hour salt spray protection I have some so I plan on giving it a try Oh yes it is most assuredly not cheap.
I use his Paints almost exclusively.Plus for removing rust I've found nothing better than the Rust Blast