American Airlines have never painted their aircraft since metal fuselages came along in the 1930s, apart from the A300 which requires a special coating. It does save weight, cost, and the downtime when repainting, there seems to be a bit of extra maintenance in return. A few others (as here) do the same. It seems to be sufficiently minor that it's down to the corporate identity whether to go for it or not.
AA uses a special polish that is made for them from a shop in Georgia. I had to cal this shop to find out what I could use to strip the polish off when I was the project manager on a couple DC-9's they sold. The shop would not tell me the chemical make up but did tell me what would cut the polish. So the man-hours spent keeping the aircraft polished does hurt on the return of investment of not carrying the extra weight.
Solids remaining after. I understand all aircraft are accurately weighed both before and after a paint job...
Not always...there are mathematical methods that can be utilized to determine the new weight. Depending on the paint scheme, the old paint thickness is measured...usually 4 to 8ml thick. Then the new paint thickness is measured and mathematically the weight is found and the difference is add to, or subtracted to the Basic Weight of the aircraft.
Regards,
Greg