I use Neutra Rust 661 and can thouroghly recommend it, very easily applied, very liquid so runs into the crevises, actually needs rust to activate,and can be painted over direct with either waxoil underseal, hammerite or paint,
used it on a Very Rusty Rover P6 sold said vehicle 3 years later and still very much rust bubble free,
I do not work for the company I hasten to add...........................
Still using it on my Landy........................1 litre goes a long way...................
Used on North sea oil rig Legs apparently