If the original paint is in good shape and completely covers the metal, your aluminum respray efforts can be greatly reduced. If there is no flaking, peeling paint or bare metal exposed, you will eliminate many of the bare metal surface preparations, usually involved when coating aluminum.

If bare metal is exposed, it will have to be spot primed and blended into the original surface. Each paint manufacturer sells a primer for their products.

Basically all you have to do is lightly scuff the surface with a non woven pad, (scotch brite), clean the area with a degreaser such as Prepsol or an equivalent, and depending on the top coating, first spray on a conversion coating. Marine and aircraft products are the best, and are similar in pricing to the top of the line automotive products.

Most of the expense in a respray is the labour involved in the preparation and the materials' costs. The project can be a DIY project, having a pro only apply the paint as the last step. An inexpensive HVLP spray out fit can be obtained for ~$100 that is adequate for this type of work. It would completely pay for itself during one project.