Weathering is a hard subject to describe, but heres what I am beginning to do and I get the look im going for.
I take my completed skin and add another layer above everything except my color variation layer (see above reply). I then take a white, 1 pixel wide brush and go crazy. I make streaks everywhere on the skin, no matter what direction or length, just everywhere. Then, i take an eraser tool and turn it down to around 20-40% pressure and run it randomly over the layer. I just pretty much wave it back and forth all over to give each streak (or what they are, scratches) a custom look so no two scratches are alike. Then, i turn down the opacity of the scratch layer until I see what i feel looks good.
If you look at the standard spit5 skin youll see the scratches all over. Thats what happens with the method I posted above.
Hopefully this helps.