A portion of my homes 40 year old aluminum siding is discolored and needs to be repainted What is the proper way to prep and paint it?
I used primer that claimed suitability for chalking finishes and a topcoat of high quality latex. It held up well for 15 years with one refreshing coat along the way. I had considered washing with TSP, but decided it wasn't necessary.
In your question you use the word "repainted", so the assumption would be that there is an existing coating of paint.
If the existing paint is sound; that is, no peeling, bubbling, crazing, alligatoring, etc., than it can be directly repainted after being washed/cleaned thoroughly. If the existing paint has any sheen (gloss) then it should be deglossed (by sanding, or chemically) prior to cleaning and repainting.
If the existing paint is not sound, or if any of it peels or is discovered to be unsound during your prep work, than primer (for aluminum) should be applied (after previously mentioned prep work) prior to painting.