I'll start with a basic gradient layer, much like what GiantFlyingRobots does with his three layer treatment. I'll then hand-paint in some very soft color lines using light blues and tans to get some color into it. Add in an extra low opacity layer or two of white and black over some panels to get them to look like they aren't from the same piece of metal.
Here's where my tricks come in.
I'll add a layer using a stock image of real metal and lower the opacity to less than 10%, grain merge to rough it up.
Add other layers as you see fit to add more texture to it. I'll often use textures of dirt, rock, old leaves and other assorted junk, desaturated, to further break up the texture.
Another layer or two of very light gray lines to scratch up removable panel edges, around screws, high wear areas, etc.
For the polished metal that I showed early on in my Mustang thread, I looked at a couple of photos of some really shiny mustangs and hand painted a layer to look like they did. It took lots of smudging, blurring, sharpening and spray work with a soft brush to get it to look OK. Then I lowered the opacity and set it to Grain Merge. I still need to work on getting that shiny effect to look right on painted areas, however.
Linear Mode
