I've just received my first 8,8cm FLAK 18 round and while the outside is in good condition I notice that the inside of the head is quite rusty. It's obviously not in plain sight but I would like to conserve it, to stop any further rusting. What's the best thing to do - leave it alone, or fill it with something like Evaporust until the rust goes then treat it with oil or something? Any tips from experienced hands here most welcome!