My thoughts:

Using a roller in that situation would not be a problem, but every time I have tried to spray outdoors, the amount of paint drifting away has been just unbelievable. I'm lucky if half the paint goes where I want it. So you're gonna end up using a lot of paint and also painting any other car within 10 feet of you.

Also, if you use the roller method, and then sand the paint smooth and buff it out with an electric buffer, there's no need for a clearcoat. As far as I know you can't use a roller for clearcoat.

Finally, the roller method also won't work for metallic paints, because they require a clearcoat (see above).

So what I would suggest is, using a roller and avoiding clearcoats and metallic paints if you want the job to turn out right. Unfortunately, this thread is not about spray-on paint jobs, so not much of the info here will pertain to using that method. Not that there's anything wrong with using that method, but this thread does not really discuss it. Also, I don't think Rustoleum is formulated to be sprayed on (except for the kind that comes in a rattle can) so that's another reason not to run it through a sprayer.