My $0.02
There are some rattle can sprays that have the real stuff, but the atomized overspray (@80%) will endo up all over your car and interior and in the garage. Best to buy a compressor (new or used) and a spray gun that uses HVLP technology (something as good as or better than a Devilbiss Finish Line gun).
If you are spraying metal and have/had rust, then the metal should be etched with a metal prep or an etching primer. Painting a car is science and chemistry and attention to detail and TONS of prep work. Letting a car sit under primer is OK, but be aware that oil from your fingers, natural oxydation, air-borne microorganisms and other factors make a primered car harder to paint after about a week.
Want more info?
check out this link and ask away...
http://www.autobodystore.com/cgi-bin/config.pl?index
Good Luck
Joe