I tried to spray paint a bike frame with spray paint from the hardware store. Actually, I did the preparation and persuaded my brother to do the painting because it was a winter camouflage pattern and he is good at that. Anyway, I can report the following results.
I did not put any sort of clear coating over the paint because I was worried about compatibility with the paint, so the finish is just paint. It does not look as good as a powder-coating job.
After a winter of riding, there is some rust. In other words, the home painting job is not as durable.
If the preparation is not FLAWLESS, then, when you hit an imperfection with primer, like a little old paint or decal glue not TOTALLY removed, the imperfection will just JUMP out at you when the paint hits it. Preparation is everything.
The only reason I did it myself is that I wanted a winter camo pattern and this was the only way I knew to get it. The result is good, thanks to the artistry of my brother. But for a single-color paint job, if you want it to look good, I would reconsider the plan to do it oneself. YMMV.