Forgot to say that he would prime the bare wood in the few places he sands.

The older coat of pain is the primer .. he only need to prime the places he sands to bare wood.

Thanks lazygardens. That's what he said but didn't know if it was true.

I suppose that sanding the entire house would only be necessary if you were not washing the house first. Sometimes for whatever reason power washing is not feasible. You just don't want to paint over dirt.