just like silverspeed said wire brush of all the lite rust and waxoyl the lot, i do my car every summer put it up on axel stands take off all the wheels arch liners and clean and reseal. i found not to use the pressure tin, the sprayer gets blocked as the waxoyl cools and things tend to get very messy and it makes thins *BLEEP*ty when you need to remove them(ive learnt from experiance) what i do now is put a concrete slab or a few bricks in the oven heat them for half an hour ,then if you have an old metal baking tray that the mrs doesn't mind you using!! put the bricks underneath the baking tray to keep the waxoyl nice and runny, oh yeh warm the oil up as well
this will keep the oil workable for about 2 or so hours possilbly longer on a sunny day plenty of time to do all of the underside of the car.
to apply the oil i use small foam rollers from a cheap shop and a radiator brush to get underneath pipes and petrol tank, i also do all the suspension joints nuts and bolts and the brake hubs as well its amazing how much difference it makes when you come to take something of 2-3 years later and theres no corrosion to foil your plans, hope this makes sense started this last night while drunk and now finishing of hungover