Racers who "blueprinted" blocks used to use a product called glyptal, which is an electric motor internal case coating.
The only reason it was used was it made it easy to wash out debris after blowing engines up, and it sealed the casting pores so casting sand or casting material weeping out of the casting pores couldn't contaminate the oil, it wasn't used as a rust inhibitor, and probably not practical to use on a street engine.
I wouldn't use any conventional paint on those covers, if it starts to come off, it will really be a mess in your oil system!
