1) by bruising the brass, does this mean the surface of the brass is depressed below its level before the steel hammer hit the brass?
Yes, so it is crushed in the highly localised area to a reduced wall thickness.
2) Does the urethane layer change the force of the impact or does it distribute the force more diffusely to minimize the bruising?
Both. It does not bruise at all. The pressure in the area of impact does not exceed the compressive strength of the brass.
However it does exceed the
bending stress, which is what we want in order to help bend the metal back where it should be.
3) Where do you get your urethane sheet? I bought mine from Kraus.
They do not sell to the general public, only technicians http://www.krausmusic.com/handtool/hammers.htm#632C
Even if it is not listed, Musicmedic may supply it. (Very helpful firm!) Or you could order it via your technician.
4) I guess that's short for polyurethane sheet? There are many "formulations" of polyurethane, probably top secret. It comes in different hardnesses.
I am only
assuming from its properties (and colour that it is probably a urethane. I may be wrong.
5) Is there any significance to the "Red" color?
Probably one manufacturer's code for the durometer (hardness). This one was particularly well chosen by Kraus, an engineer by trade.
6) Do you glue this to the hammer with contact cement?
Yes. It needs to be a really good contact cement, not the sort that "creeps". It's a challenge over a curved surface.