As Mike said the skin comes off fairly easily if you are careful. It's the door frame you don't want to damage. We used the skin off a good door for a pattern
. The rust you see on the inside is just from the filler crap that fits between the skin and the frame. Using a whole skin for a pattern helps to make the overlap spots the same on the corners etc. Careful use of the hammer and dolly flattens the skin right out. We used 20 gauge as it is so close to the original and is heavy enough not to oil can or warp. 18 gauge is harder to work and you have quite a curve to form. We use satin coat because it is more resistant to rust especially on the inside. Scratches in normal sheet metal start to rust very easily, even when sitting around the shop. Satin coat is now $48.00 a sheet.
Barry