FWIW, I used to have a section of my frame covered with a small section of inner tube to protect the frame from getting chewed up by the clamshell style bike racks we had at work. It did a good job of that, but I noticed one day when I removed the inner tube that it also discolored the frame. That didn't bother me because it's a 20 year old beater bike, and I still prefer a little discoloration to exposed steel and rust...
