I believe you should not worry about external rust too much if the cavities are good and the top of the molds and the sprue plates are good. You really don't want to remove the internal heat oxide from the molds as this is an aid to casting performance. My feeling is that as you use the molds and oil them to protect them between sessions the rust will gradually slough off from the preservative oil. I know they may not look very good good but if they still work I'd leave well enough alone.