I have painted two frames, one rattle can (using duplic color) , one using automotive finishes with a preval spray can

My experience (from this limited sample)

you can get a great looking job from rattle can, but it will not in any way as durable as automotive finish.

Rattle can will cost more than you think (at least for me who was king of thin coats and lots of over spray, but no run)

Automotive is a lot more expensive again. (partially be cause of quantities)

as noted prep is key..... this is my quick list of steps

frame to bare metal.....use chemical stripper

use an acid etch primer to seal

use a high build up primer

sand

2-4 thin coats of color

3-4 thin coats of clear

pay close attention to recoat times

and adding use a good respirator it is a critical

ymmv