Next thing to do is to go back to the paint layer, (make sure you select the layer & not the mask.) & add a lighting effects filter. Feel free to experiment with different light types, colours & combinations until you are happy. Just remember that sometimes less is more. Remember to drop the menu down where it says texture channel & choose Alpha 1. You should also have ' white is high' checked.
Here's the settings I used.
Now go to edit/fade lighting effects, (or press shift-ctrl-f) & reduce the opacity to about 50%. Leave the blending mode as normal. I had it on darken for a while, but it lost some of the definition due to highlights. I found the other modes changed the hue or contrast too much. Again, experiment to your own taste.
Now select the mask & with a fairly small soft brush, (17) on about 10% opacity, paint around the edges of the dents and also in the middle of them a little. Use the blur & smudge tools if you go overboard. What you're doing is making it look like the paint has got scratched off & worn where the dent is. Now with a very small brush on about 50% opacity, build up some scratches around the dented areas. Also add some faint random scratches elsewhere on the texture too. Another trick here if you find you've gone scratch crazy is to duplicate the paint layer. I kept it on normal blending mode at 100% opacity, but as always play around to see if you like anything else.
This is where having a pressure sensitive tablet pays dividends. Using the same brush & settings, but different amounts of pressure, you can add lighter or deeper scratches. Lots of little scratches close together make nice scuff marks too.
Now to add some chips and cracks in the paint. Obviously there are any number of ways to do this, but this is my method. Its important that you read this bit carefully first before you do it. If you are unsure, copy the 2 paint layers & hide the originals by clicking on the eye next to the thumbnails in the layers tab. Now work on the copies. If all goes well, delete the originals. If not, delete the copies & start over.
Open up a decent grunge map & drag it into your texture as a new layer. Use free transform to resize it to fit if necessary. Now, using the magic wand with a fairly low tolerance, (I used 5) select a part of the texture according to brightness. Usually I go for whichever has less, black or white. Don't worry about shades of grey so much, just pick the lightest or darkest colour you can see. Now go to select/similar. You should have all of the black or white area on the grunge layer selected. Now hide the grunge map layer. Without altering the selection, click on the lower paint layer's mask. Using a large brush on about 50% opacity, paint over the selection (basically the whole texture,) with black. If you've done this right, you should see areas of the metal showing through. The black simply adds to the masked area of the paint layer, allowing the layer(s) below to show. Now repeat the process on the second paint layer's mask with a different grunge map. If you only have one, then use transform under the edit menu to flip the layer. Vertically, horizontally or both - it's up to you. You could also add rotation or whatever you like, just make sure you get a variation on the original. Now I used the same selection again on the lower paint layer's mask again, without flipping or anything, just to make those spots really show through on both layers. If all went well, you should have something like this:
Of course you may need bigger chips or finer detail - just size your grunge maps accordingly using the free transform tool. You don't even have to use the whole map. Size it up so you can only see a small part or add & subtract to & from the selection. You get the idea. Once you are happy you can delete the grunge layers if you want, as you won't be using them anyway. You can also add to the masks by using a speckled brush. I added a drop shadow effect to the top paint layer at about 20% opacity.
Lastly we'll add some rust. This could also be mud, sand (used in conjunction with the noise filter) or blood spots - whatever. When thinking about rust in particular, remember that rust is generally caused by water damage. This means that usually where you find rust, you will also find rusty streaks down the object where the water has gone. Rust also attacks exposed areas, so it is more likely to occur on scratches & dents etc, than areas where the paint is still intact. Create a new layer & set the foreground colour to 100,50,20. Now paint in areas of rust colour on the new layer, keeping opacity low.Just go for broad strokes at the moment. I used the smudge tool to create streaks& then added a Guassian blur, 5 pixel radius. I then used the dodge & burn tools on 10% exposure to highlight & shadow certain areas. Next I duplicated the rust layer and painted in some sharper streaks using a 17 brush, also using the smudge & blur tools sparingly where I thought it was necessary. Normally I would spend a little longer on making the rust streaks look better, but it's very late, I'm tired & you get the idea. Anyway, the top rust layer was set to hard light at 80% opacity & the bottom one was set to darken at 35%. Obviously you'll want to play with these values to suit your needs.
Also by changing the blending modes on the rust layers, you can achieve different effects. Multiply gives you mud, screen is sand & colour burn gives you oil. I'd usually add some noise or other effects to these, like plastic wrap to the oil & maybe mud to give specular highlights. (Actually I just tried this & it didn't work, but you could paint then in by hand using a soft brush & white. As before, broad strokes with some blur & then add tighter highlights on top.) I did add some noise to the sand layers though, making sure to ctrl click the layer to select only the painted areas.