Step 6: Open up the texture from the last part of the tutorial. Go ahead and flatten all the layers. Here's a way to make reeeaaally easy scratches. Click the pencil tool and draw a buncha random lines to look like scratch marks, I used a medium grey color
Step 7: Duplicate the layer and set it to multiply. Click back to the original and nudge it down and over 1 pixel
Step 8: Drop the opacity of the original layer down to about 70%. If necessary make the top layer a little darker. Take the eraser tool and wear down the edges of the scratches just a bit, so it's not so harsh. Run a noise filter on the lines if you want.
Step 9: Here's one way to apply slight corrosion and rust. Open up your original metal texture. If necessary, flatten the layers. Duplicate the background and flip it vertically. Set it to "overlay"
Step 10: Select the eraser tool and hack off a good 2/3's of the top layer. Be sure to follow up with a "grungey" shaped brush to make the edges uneven. Reduce the opacity to about 75% and up the contrast about 65%