By "bonnet interior" I take it you refer to what us yanquis call the "hood"...
Rot-oxide primer under there? Nooooooo! That would have been a classical automotive sheet-metal part, not some piece of heavy plate steel or other stuff like that. It would certainly have been "dipped" in some finish color (recall that, unless this were a replacement "hood", it was made and installed on the original track and was almost certainly a fully-finished item up to automotive manufacturing standards). (This is the same response one would get from me if for instance they were thinking of painting the underside of their Opel Blitz or Steyr or Mercedes truck bonnet in red primer. Same thing, done the same way. Very most likely a fully-painted, finished item. An auto part. NOT like a late-war StuG or tank, or the later-added armored cab of your track, where heavy metal was welded together, primed, and in some cases, painted or not with base coat and camo.)
Now, if you were talking about those cast service hatch-covers on panzers... I suspect that those were red primer inside, specially so on later tanks!
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Bob
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