Another response to "How to remove them?" Brake cleaner. I was working on my truck's brakes one day. After I was done went to work on my bike. Grabbed the wrong can to start degreasing my chain. On touch of the break cleaner, and the chain gaurd and reflector just shattered. Scary stuff, just like Off. The same stuff we use for repellin misquitoes can remove Marks a lot markings from paint and tile.