Yes, probably 4 or 5 pax worth of weight on a 747. Maybe more, it was 3 or 4 on a DC10.
Corrosion, I think there is a fine lacquer on to prevent ally-scab.

The real penalty is the butt-ugliness. Unless all the skin panels come from the same batch from the metal mill, you get a different shade, as can quite clearly be seen on their a/c. Working the metal also produces different shades, a flat panel with a single curve (Fuselage) will not have the same shade as a double curve panel off the empennage/nose.
AA used to insist that the inlet L/edges are made from the same batch, especially when there are two lumps hanging off the same wing.

I seem to remember that the first few A300’s that AA got were very splotchy due to the procedures that Airbus was using at the time, therefore they had to be painted. Apparently, no other customer had specified (demanded) bare finish up to that point in time.

The only reason I remember it is that Av. Wk. had a blurb about it and said that the color that AA specified was “Boeing Gray.” Can’t vouch for complete veracity of the original story, but I do remember seeing it in print at the time and thought it was entertaining. 🙂

Airbus apparently came up with a solution, as the AA A300’s are now bare. I also seem to remember that Eastern had painted A300’s, but apparently they didn’t make a big deal out of it with Airbus or in the press (the got them long before AA and allegedly got the first 6 free, maybe they didn’t have any leverage?). I also remember seeing 727-100’s that were painted at Eastern, and asked why and was told by a cohort that they had been polished so many times, the FAA was worried about removing any more material and told them they couldn’t polish them anymore, thus the paint.