Well you'll probably gets all sorts of advice but if it were mine I would thoroughly scrape off all the loose stuff, no point in putting anything on a flaky surface.
Once I'd cleaned as much as I could with a wire brush I'd use a rust treatment to stabilise things. I've used dinitrol rust treatment in an aerosol which seems good.
After that the flat chassis sections or outside you need to paint ior apply a wax coating to keep the nasty stuff away. Internal sections need a cavity wax and again I've used dinitrol but others are good too. Waxoyl isn't the most effective but better than nothing, loads of info if you google it. Stone chip is good inside arches or on sills etc but tends to flake and allow moisture to creep beghind it on chassis rails. Soft waxes are good and kind of self heal or are easily topped up annually.
But use a mask and dress with the right kit as I still have a pair of overalls that stand on their own!
