Preparation is everything for a good finish,
First wipe the area that is to be painted with "Acrylic Thinners" This is to get rid of any grease, finger marks etc.
Then paint the metal surface with a "Metal Primer" this gives the paint a "Key" although some people say metal primer is unnecessary, me I would never paint metal without first applying a metal primer,
Next If you have access to an Airbrush then use it, Rattle cans and the hairy stick are very much second rate alternatives,
If you have to use a rattle can make sure the can is not cold and do shake it robustly for at least a minute before using it, and ensure the area you are using to paint in is free of dust and is well ventilated and has a non flammable heater to aid the drying process.
shaun.