Actually, to make the job last longer, we do 4 steps:
1. Lightly rinse the aluminum siding and apply, by spray a mild acid. The acid loosens the old chalky paint and opens the "pores" of the subsurface paint. 2. Apply a high alkaline detergent and pressure wash with hot water (about 170 degrees). The old paint will wash off. However, there will be paint residue at the base of the wall. 3. Usually the remaining paint will not require a primer since it is now free of any pollutants and is ph neutral (acid wash / alkaline wash....the 2 offset and become neutral - if in doubt, rinse and check the runoff with litmus paper). 4. Using an airless sprayer (much faster than brushing and you can rent one) we apply our exterior latex. I suggest using the paint mixed with Floetrol (you can purchase it at most paint stores / Home Depot / Lowes). Floetrol makes the paint flow on to the surface much more smoothly and you don't have to clear the spray er tip as often. Works well if you are brushing / rollering - goodbye lap marks!
And, for the ultimate in long term...apply a clear finish coat. We have used a clear urethane with a lot of success. Before applying the clear coat, allow the paint to dry 24 to 36 hours. Rinse or brush the surface to remove any airborne pollutants before applying the clear coat.
This should allow the paint job to outlast the original baked on factory finish.
And for those of you with new siding, rinse and apply a clear coat as soon as the siding is up. It will save you the job of repainting 15 years or so down the road.