Hi Diesel, I don't do metal moulds (yet)
but i do do Resin and Silicone.
I'll wager that its a piece of string question, and dependant on a lot of factors. I.E. what size mould cavity etc.
I'll put money on using toothpaste,auto sol. or other metal buffing compound with a cloth or polishing mop. rather than an acid though.
I'd be tempted with a "dremel" type tool (without going into the bearing debate lol.)