If done correctly it can come out quite nice looking. It'll never be a strong as powder coating or as durable as anodize, but the end result can also be touched up by the average guy in his garage.
As stated, prep is key.....don't do anything until the surface is ready. On aluminum I recommend not only getting rid of any anodize, but to beadblast the surface so that it appear dull to the eye and feels the same. Once there (again as stated above) use automotive grade etching primer. take your time here and get a nice base for the paint. Let the initial coat of primer completely dry before going on. The use something like krylon grey primer as a 'fill' spray and sand until it's very smooth and then you can paint. Follow the directions for the paint you'll be using and you should be fine.
As for paint....you get what you pay for. if you buy crap..then end result will be the same. When I've done it, I've always used high quality auto paint purchased from an automotive body/paint supplier...not the local supermarket 'touchup' garbage.
For an example go here:
https://www.pbnation.com/showthread.p...hreadid=347538
A note on durability. Cars are baked at between 160F and 200F when they are factory painted. This helps to harden the surface and increase adhesion. You might consider doing the same. DO NOT bake a painted tank however since you can anneal out the strength and cause a potentially deadly explosion when it's filled.
If you blow yourself up...it's you're own fault!