if you're going to use a rust eate, just go into a paint factor's and get them to mix up an aerosole of 'etch primer'.
On production lines, it's the second stage dip after galvanising, and it preps the surface for paint, and it's what's used in bodyshops for bare metal prep.
Oh and if you do go for it yourself, don't wait two weeks for the paint to dry before laquer.
Clearcoats are designed to go on wet on wet, meaning that after letting the base colour coat flash off for 10-20 minutes, then you go on with the clearcoat - normally two coats of base, two of clear.
ideally if you want to keep costs down, you might want to think outsdie of the box and find a college that has a paint refinishing course, you'll only pay for materials then, and it'll be a half passable job.
last of all, you might want to consider masking the back of the door visible in the arch, and spray a rubberised stone chip protector on it for added protection in this vulnerable area.
before anything though, wash and cut the paintwork back thoroughly, you need to have the oriinal colour as much as possible showing otherwise your colour match is going to be miles out!