Step One
OK, first off we create a canvas, mine is 450*450px, with a white background. With this open, create a new layer (layer–new layer) and grab the lasso tool (l) - now go wild, make as random a selection as you can, and fill it in black, mine is shown.
Step Two
Now, use the wave filter with the following settings:
Wave
Number of generators: 5
Wavelength: 10-120
Amplitude: 5-35
Scale: 100%:100%
Type: Square
Undefined areas: Repeat Edge Pixels
You should have something like this.
Step Three
Now, use the find edges filter (filter–stylize-find edges) to get something like that shown.
Step Four
Next up, use the chrome filter (filter–sketch–chrome) with these settings:
Chrome
Detail: 4
Smoothness: 7
To get something like that shown.
Step Five
Finally, use the glass filter (filter–distort–glass) with these settings:
Glass
Distortion: 5
Smoothness: 3
Texture: Frosted
Scaling: 100%
Invert: No
And you’re done - pick out whatever parts you wish to use as grunge brushes!
Conclusion
Here are some brushes I extracted from the image - these come in great as scratches on metal if you use the darken and highlight tools - have fun!
