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I think the problem is usually people not treating like a proper spray job and ease of access. A rattle can is just a single container with the paint, thinners, and air pressure all in one. All the techniques required for a good spray gun job are still required for a rattle can job. There's also an element of pre-selection. People with the motivation to invest in a full spray gun set up are less likely to knock out half arsed attempts.
The other big hurdle is inherent difficulty spraying a bike frame which is made up of lots of overlapping tubes. It's not like say, a car wing, where you've got one nice big block to go back and forward across.
Pretty sure either Spotter or a Dutch guy on here did a good job with a purple bike.
The thing is you'll probably spend ~£100 to do a good DIY job (assuming you're buying everything from scratch).
hugo7
FWIW I've never seen a good rattlecan paintjob.