Almost sorry to bring this back up - BUT.
Just removed the cab from the '48 2 ton farm truck from New Mexico. It had a nice even fine rust finish with some chassis black showing in out of the way places.
Decided to power wash all the silt and grease off.
Discovered as I passed over the outside of the frame rails, that they were getting blacker. I worked an area including some riveted parts and then inspected surfaces, edges and rivets.
My opinion as a Design Engineer with almost 30 years experience - This frame was never painted. It was treated with a chemical surface finish after riveting and before accessories were bolted on.
If I had blasted or done any other abrasive cleaning I would have not found this as It would have been taken down to bare metal. The surface rust formed because of microscopic bleed through but the surface had not broken down.
Can only answer about this one frame - no generalizations.
