From my experience, with metal or 'plastic', clean and rinse with dish soap and water.(plastics mainly). For metals that are smooth or rough textured surfaces, first soak in vinegar for a few minutes and then rinse in clean water. The vinegar is a weak acid that microscopically etches the surface, plastic or metal. Prime the surface with an enamel aerosol primer of either white, gray, or black. Now you can use any paint media you wish. Best to use enamels or lacquers first for base colors as they are tough and can tolerate the 'working in' of subsequent color media. After base color is dry, use any media, acrylic inks, water colors, enamel, lacquer and apply detail colors you wish to be prominent to the eye when finished. when this is dry, seal coat with either gloss or matt clear enamel or lacquer. Now apply filters or washes for special effects you wish to have to either 'age' or 'gem' the detail. When this dries, seal with gloss or matt clear enamel or lacquer for toughness and wear. Practice,practice, practice, until you get to know the media. Primers are good to go after 20 minutes of dry time as a general rule. Humidity and temp affects all color medias from primers to inks( either alcohol or acrylic). Lacquers and oils take about 7 days to mature after application. Enamels about 3-5 days, and acrylics and inks 1-3 days. Again, process test pieces as you go along to determine when the next step is good to go. Be Brave, smart, and confident.
Note: it's good practice to let each wash or filter dry before sealing and applying the next wash or filter.
Methods of application are entirely up to you. Experiment to your satisfaction.
Have fun always.
-R