My local lumberyard called their paint supplier and they said if the paint has not completely faded from my factory baked steel siding that I do not need to prime, just paint. My common sense is telling me I must prime even if it's over the old paint. Does this make sense to anyone? I can see their point of view- why prime over paint? The paint is faded significantly on one side of the house (so of course I would prime that without a doubt) but the rest is fully intact. I did already scrub the house with TSP so it's ready to go.
I'd still prime it. Since it's never had anything but the factory finish on it, priming certainly can't hurt. I'd put an acrylic rust inhibitive DTM primer on it to help head off any possible rust problems in the future and top coat with a good quality DTM as well.