Here are picturs of the frame interior of my 1914 LP-08 Erfurt. Not the best pictures, but they will serve. The inside of my 1911 Erfurt is as white and clean as if it was made yesterday.
I don't remember where I read this, but...rust blued Lugers were blued in several stages, the rusting chemicals applied and the rust carded off several times before the finish reached its final depth. The interior of the gun was polished white after every applicatioin.
--Dwight