I believe that underseal may have been optional at some stage? That photo looks like a galvanised chassis with thin brush applied black paint. as Mark pointed out, the black Dinitrol is super effective but difficult to cope with when you need to dismantle things.
I wouldn't use a wire brush on the galvanised surface. Try using a rag soaked in white spirit and see what remains. Then you have a choice of applying Dinitrol, or Tectyl 506 is also excellent.
