Great information! If I'm ever going to buy a gun I'll ask you first.
Sumo will probably know what's the finish on Finnish assault rifles (matte black) that finish is really strong. I never saw rust on them.
The Valmet series used a park solution. Very durable as the park crystals hold oil. Matte black generally means a blasted surface is blued, it will hold oil but the bluing is still a rust process even if it is controlled.
