Primer paint must be applied to clean metal. However, it’s not enough to just wash the bare metal with a good cleaning agent and water. You will need to ensure that the metal is as free from oxidation prior to paint application as possible, otherwise the paint won’t stick properly and the metal will begin to rust immediately. To properly clean metal, you need to wash it thoroughly with a good cleaner/degreaser and dry it immediately. Once dry, you need to immediately apply at least a light coat of primer to it to keep it from oxidizing.
For more info, see Self Etching Primer vs Epoxy Primers