The key with anything to do with skins is layers, layers, layers.
I probably have way too many, which tends to mess things up a lot, but I have seperate layers for almost everything... camo, rivets, panel lines, markings, dirt, exhaust stains, paint chips.... everything.
This way you can adjust the opacity or whatever else you need to do for each detail independantly.
One example is having a seperate layer for panel lines... You can load the panel lines (so that just the lines are selected) and (on a new layer) create a layer of dirt in the exact shape of the panel lines... Gaussian blur it out a little and drop the opacity and you have nice, crisp panel lines with a layer of smudged out dirt over the top of them to help give them a little depth.
This saves tracing a layer of dirt over the top and gives you a crisp following of the lines.