While I think it is a bad idea to paint surfaces inside a cooling system component, there is a paint for such use called Glyptal ..............I think.
There was a recent thread dealing with an engine rebuild where it was used inside the water jackets.
Thanks ccjersey I was a bit worried about the heavy pitting in the bottom housing, so I thought painting it might preserve it but I guess the corrosion inhibitor will have to suffice.
Regards
Phil