1 Prep and clean all surfaces, using wire brush, scraper and ideally steam cleaner. Try and find a company who steam clean buses etc and get it on a ramp
2 Use a rust convertor which eliminates any surface rust. Coat chassis rails and other easily accessible areas with a 2 part paint solution, ideally, Rustbuster EpoxyMastic 121. POR 15 also make some good quality products.
3 Use Dinitrol (can be sprayed) into harder to access areas e.g. cavities etc. This is widely used by the MOD in the UK on their Landrover fleet. This meets their spec which states that their vehicles must be able to be driven in 1m of salt water.
Don't waste time with inferior products, some of which are market leaders, unless you want to re-do the work every year!
Think outside the box