Thanks for the replies...I am familiar with how layers work (I use photoshop CC by the way) but the real problem I'm having is actually painting the wear and tear, and having it look realistic. A lot of people I hear are painting brown/gray spots and using the smudge tool to make it streak across the surface. However when I do that, the smudge tool makes some strange pink, green and purple stretching artifacts on the image. It's hard to explain, but unsatisfactory nonetheless.
I don't know...maybe I do need to just keep piling on the layers and something good will come of it.