IMHO, if you have an air compressor, the best cheap paint coverage for the money and decent end result will be a cheap HVLP (High Velocity Low Pressure) type (HF has one for $15 and coupons often for less than $10 that works just fine for cheap) and primer and paint from your local farm products store like TSC. The paint they have for tractors and equipment is very durable, lays easy when thinned properly for the temp, humidity, etc and they are really reasonably priced. The colors tend to fade pretty quick if left in the sun all the time but if your looking to cover a decent area, quality, durability and a nice finish at a low cost that's what I suggest. You can add a clear coat that has good UV protection to help with the color fade. Just make sure to drain the water from your compressor tank before spraying and I would suggest looking into how to prevent water getting to your paint gun to prevent water ruining your work. So in short: Acetone for cleaning prep, HVLP for a versatile quality gun that's adjustable, farm equipment paint for cheap durability and coverage, knowledge and practice for the best results and always primer... Any primer is better than no primer at all and will always make things better and easier but never the other way around unless you spay too much so that it runs.
If your looking for techniques then my advice is: clean and prep your surface is key to quality, long lasting results. Primer is a must. Keep the right distance and spray pattern to prevent running. It's better to do multiple light coats than trying to do less coats of heavier lays. Wait for the last coat to loose its wetness and become tacky before shooting your next coat but for the best results you should avoud letting the previous coat dry completely. The layers bond and you have a better finished product if all the layers are bound together, i.e. you shoot the next coat when the last coat is still tacky but won't transfer to your finger when touched.
If you are just looking for a decent cheap paint job and you don't have an air compressor then I suggest taking your time on cleaning and prepping the surface, laying down a nice layer to primer and be ready to shoot your paint while primer is tacky since primer usually dries fast. And then do a very light and quick first coat, allow to become tacky and then follow up with a regular layer of paint. Prep is very cheap but leads to the best results in the end no matter how cheap or high dollar paint that you use. Primer is important to smooth out the small imperfections as well as become a layer that sticks well to the metal as well as a much better surface for paint to adhere to over just paint to metal. And then from there it is just being patient by laying down a light coat, let it become tacky and then add your heavier layers on top of that.
Those are my thoughts. Hope it helps.