This is an interesting thread. I'm collecting my chrome from Ashford Plating on Monday and will paint the underside of the front and rear mudguards, the chainguard (although flung off chain lube will help keep that rust free) and the underside of the points cover to stop the potential arcing from the points.
I remember froma previous thread posted by Ken (Oddjob) a while ago where he mentioned about rust forming in the rolled edges in the mudguards.