Phosphating is a waste of time it has a maximum of 24 hours NSS salt spray resistance (NSS = Neutral Salt Spray Test = standard for corrosion resistance)
100 hours NSS is equivalent to 1 year in real life, so you would be changing phosphate parts every 3 months. The Marine, Oil & Gas and rail industry either utilise Stainless or Exotic corrosion resistant metals or they are coated in heavy Zinc (Sheradizing) finishes or heavy organic / inorganinc finishes including Aluminium Flake
Most chassis items (Under Car or exposed areas) have to meet at least 800 hrs NSS with some new finishes claiming to achieve 1500 or 2500 hours NSS
Phosphating is generally used for internal engine components or for internal trim parts that are not cosmetic. In engines it is used to control torque tension on cylinder head bolts
The trouble with the higher NSS specification finishes is the majority are non sacrificial i.e once they are scratched to the bear metal the corrosion resistance is removed / impaired This is why the photos look so bad, it is the abuse the parts have got (stones, grit etc) that has disrupted the protective layer, the road salt just helps to accelerate this once the damage has occured. On the other hand Zinc Chromate finishes are sacrificial and this relies on the Zinc Corroding before the steel substrate, therefore when a zinc chromate part is scratched the zinc corrodes and the corrosion by products fill the scratch thus extending the corrosion resistance these types of finish will achieve at most 500 hours NSS.
Back to phosphating, this is just bear plated zinc or manganese without a chromate conversion layer, as such it is very porous and corrosion starts on the base layer straight away, but a slower rate due to the Zinc or manganese corroding.
So for if i was cleaning the bits under my car i would:
Remove the heavy castings (Brackets, Wish bones etc), have them pickled or shot blast to remove the rust and then either have them galvanised or powder coated.
Fasteners or fixings I would buy in either a delta, dacromet or dorreltech finish, these are all 1000 hour NSS finishes and will be OEM specced finishes, so i would buy the replacements from the Dealer not Screw Fix or B&Q. later BMW finishes are Zinc / Nickel which is also between 800 - 1000 hours NSS and this has the sacrificial coating of zinc covered by an inert coating of Nickel, can get a really good black finish to these.
For your large panel items I would just clean away the rust and treat with proprietary paints like hammerite and wax oil over the top to repel moisture. The under body panels were painted the same as the car (7 layers) plus an underbody seal.
which I am now attempting to remove with a wire brush. 
