I do a fair amount of bike painting- mostly for my own needs. None for profit, so I'm always looking for the cheapest and easiest way. I have compressed air, an exhausted spray booth, and several spray gun types. I've painted more than a few cars in the past forty years. So why wouldn't I just use this method for a bike?
I'm too lazy to do the necessary equipment cleanup for such a small job. Automotive paints are expensive and not readily available in small quantities for bicycle frames, so you always end up with stock and not necessarily in the right colors. Then there is a shelf life so waste is an issue. Consequently I'm always looking for the right "spray bomb" combination that is reasonably durable, and looks good.
Beyond good prep, primer is key. The best available in aerosols is 2k epoxy primer. The rub is cost at around $30/16oz. can. There's enough for two bikes in one can, so to reduce costs it's best to have two paint jobs lined up back to back, cuz once activated it has a shelf life of 4 days at best.
I'm currently getting really good results with the following products and procedures.
For the primer I'm using Plastikote "ETCHING" primer. It's sandable and fills good. Two decent coats, with the second being wet enough that the surface is smooth. If necessary you can sand out the runs. I dry sand the next day, then let it cure for a couple days before applying color. This is a lacquer product. Adhers very well to a properly prepped surface.
I haven't used this primer on aluminum so I can't speak to the adhesion issue.
For color coats I'm using Valspar "ANTI-RUST" "ARMOR." Two coats, one dry- one wet. No more than 15 minutes between coats. Walk away. Have a close look the next day. If the coverage is total with no thin spots, let it sit for a week. Generally though that isn't the case. That's one of the problems with aerosols for bike painting.
It's a complex set of joints and an aerosol doesn't lend itself well to getting full coverage of all tubes in tight places. You simply don't have the adjustability of a good small gun. Aerosols also put out lots of overspray, because you can't adjust it. To try and reduce overspray, and waste, you'll likely end up with a couple "light" spots in tight places- or you're gonna have a ton of runs to take care of.
So... I let it cure for a few days- wet sand with some "dull" 220, and give it another two good coats. You've now given it decent film thickness for sanding and buffingand covered the "thin" spots. Even though it's an enamel product, it's still sandable. After a week of curing I wet sand with 600, and buff with a good "clearcoat" polishing compound. Give it a couple good coats of wax.
I do not put down a clear coat, as I've not found an aerosol clear that was worth beans. Once you've buffed out the color coats, it appears as if it has clear on it anyway. Clear is only for filling so you can color sand and buff. Urethane clears of the automotive variety add durability, but we don't have that luxury in aerosols.
I've tried all varieties of aerosol combos, and what I'm doing now is the most durable to this point- and it looks good. It's been impact tested as well, and holds up very well. You can get a decent job with aerosols, just not as easily as with a spray gun using auto-paints. It takes a bit longer due to the necessary curing times- but it's way cheaper. A shade over $20 will get the job done. One can of primer, and two cans of color.
If your time is worth much however- I'd go the powder coat route.