The secret, is to buy real automotive paint and lay it down with an hvlp gun, better results, less work, comparable money for cheap auto paint. And it looks better and holds up better.
Seriously I've seen the amount of work some guys will put into making Rustoleum look OK ,the same amount of work and an extra 100.00 in the deal. They really would have a good looking paint job, that would last.
I'm sure I replied earlier in this thread, but 100% agree with this ^.
Now if you're just spray bombing over rust/dents, whatever. Making a patchwork pony all the same color so it doesn't look ridiculous from across the street, fine.
If you are truly spray bombing it and don't have a compressor and gun, then there's a point where better than it was is better than nothing, but if you're actually trying to make it look good, then you can do it economically but have to spend a few bucks to get there.
Besides, if you're fixing and painting your vehicles, either as a hobby or keeping a daily on the road, investing in tools is a wise investment. Hit the Craigslist, facebook, yard sales, etc.
Heck, if you live near me and can't afford a compressor and gun, I'll loan you both. FWIW, my spare compressor I bought for $50 used and it'll pump air all day.