to ensure best results, make sure the exsisting paint is sanded extremely well before the primer is applied. if not, because it will get fairly warm under the hood the paint will flex a little bit and if the primer as not adhered to the surface due to improper sanding, the paint will chip and crack. seeing that ♥♥♥♥ on your car sucks because you will forever notice it and it requires resanding and applying paint again. so do it right, make sure it's fully sanded.
i'm going to college or collision repair and have been working in the field for almost 5 yrs. now. this rattle can idea isn't a good idea. they are not meant to spray large areas, like you said you had good success with it on small pieces. spend $50 bucks at home depot on a gun, hook it up to your compressor and spray. XO-Rust leaves a nice shine, you just have to cut it with a little laquor thinner.
let me know if you have any other questions