I've recently gotten into DE shaving and acquired a 40s SS from an antique store. I cleaned it up and while not collector quality, it certainly is shave-grade. I've used it a few times with a Derby blade, but about 4 days after I've put the blade in, I'm starting to see what appears to be rust on the blade and it has transferred itself to the razor. Is this normal for blades to do this? Given that it is stainless steel, I would guess not. Why is it doing this? Thanks.