Ok, here's what I like to do. Take some good pictures of dirty things, like dirty concrete. In photoshop turn it into a high contrast image using curves. Take out circular parts to use for grunge brushes, define them as brushes.
I can't really remember how the brushes in 5.5 work, 7 and above have a great brush engine, I think Photoshop Elements also has the good brush engine. This is a lot easier with the 7 brushes, but you can do it with more work in 5.5.
Here's my process:
1.start with a base for your texture, keep any decals on separate layers:
(url="http://"http://www.adventuredog.net/images/texbase.jpg")Base(/url)
2. paint on some dirtyness with one of your defined brushes, change the size and angle a lot, so it looks broken up.
(url="http://"http://www.adventuredog.net/images/tex01.jpg")Adding Dirt(/url)
3. erase out a lot of the dirt using a grungy brush, but leaving a bit of the edges is usually good, it makes it look like stains. Use a medium opacity on the eraser, so you can build up how much is erased.
(url="http://"http://www.adventuredog.net/images/tex02.jpg")Takin' out dirt(/url)
4. If you want your logo/decals to look worn, make a copy of their layer, hide the original, and erase out parts on low-ish opacity with a grungy brush until it looks faded and worn. Make the original layer visible again, but blur it a little and turn the opacity way down.
(url="http://"http://www.adventuredog.net/images/tex03.jpg")Worn logo(/url)
5. Add a lot of a color with a grungy brush, keep in mind where you want edges to appear, because then you'll remove most of it with a low opacity eraser.
(url="http://"http://www.adventuredog.net/images/tex04.jpg")Lots of color(/url)
(url="http://"http://www.adventuredog.net/images/tex05.jpg")Erased out all but some edges.(/url)
That's the basic way I do dirt. The best thing to do is keep changing the blend modes and find what works best, and assemble your own grungy brush library. Use different colors too, sometimes it makes for interesting stains. Look around at where dirt collects, so you'll know where to map more caked dirt.
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