These are good for slightly larger area such as garage doors, although better suited paints are now available such as garage door enamel.Just paint it as you would on any other surface. Often takes slightly longer to dry on metal than it does on wood.It is  a good idea to clean the metal that is to be painted by using a clean rag and some of the thinners for that particular type of paint, this will remove and surface dust or grease and will ensure that the paint sticks well to the metal.