Indeed, that would work, but for the custom maker, would take too long and be too uncontrolled. Generally we want to avoid the swelling and shrinking associated with wetting and drying repeatedly. Also, proper "faking aging" takes into account that some areas of a gun get rubbed down better than others, whereas others are prone to trap corrosive fouling and moisture. If you look at old barrels, they are rougher near the breech than in the middle, etc. Locks are more corroded where the fouling geta trapped behind the cock, around the pan, behind the frizzen spring, etc.