This is inspired by a biorust tutorial, though Gimp has a few things that work better with this than in PS.
First, create a new image, size really doesn't matter, but mine is 640x480 and I'll be using numbers off of that.
Then fill it with a gray. The darker it is now, the darker it'll be when you finish. Here, I'm using #8e8e8e.
Create a new layer, and select all. Now go to the select menu, and shrink your selection by 20px. Then go back to the select menu, and feather it by 20px. Make sure you're new layer is selected, and fill it with pure white. Clear the selection now, it screws with the next step. (For simplicity later, move it below the gray layer.)

Now go to your gray layer and go to “Bump Map” (In 2.3.14, it's Filters->Map->Bump Map)
As the bump map, select your white layer and use the settings here:

and hit the button.
You should get something like this:

Now it's time to make it pretty.
Create a new layer above the bumpy gray. Then go to Filters->Render->Clouds->Plasma and crank up the turbulence to about 2.5. Now mess with it until it looks good, and hit ok.
Next, with the colorful clouds active, go to Colors->Desaturate. And make sure you have luminosity selected. Set the layer's mode to “Grain Merge” and play with the opacity until you like it. Here, I have the opacity set to about 20% When you're happy, merge it down into the gray layer. (You can optionally mask it off first, to restrict it to the front of the plate)

Now the fun comes in. I (Highly!) suggest getting some grunge or scratch brushes. Create a new layer and set it to Burn (If you have the proper brushes) and use the brush tool to paint some scratches. (You can either be careful, or mask it off with the alpha of the bump map.) It's up to your artistic sense how heavy to do it.
Here's mine, with the burn layer set at 33%

You can add other suff as you wish, rivets, bolts... whatever. It's just a decent place to start.
(Also, don't be stupid like me, and save often :w: )

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