toast, instead of trying to simulate the removal of paint with the paintbrush tool, use a color selection of a photosource to get a more natural paint removal.
Like so:
Make a base texture. This is your average flat metal, made with a dark grey background and some nice photosource metal shrunken to fit properly and set to difference as its layer effect at 10% opacity.
Next make your paint as a seperate layer. Then the detail paint (text and outline) as another layer. Fiddle with the colors to suit.
The crucial step. Make your base texture's photosource metal have more opacity so that you can get a good selection with the color selector (select>color range) and eye dropper. Fiddle with the color depth of your selection, but I prefer no greater than 40 for crisp paint flake edges, so I chose 25 this time around.
Now click on the mask tool and convert your color range selection to a mask. Erase portions of it for the non-flaked away sections. Revert to selection and delete the selection from both of your paint layers.
Now it ought to look something like this. Now all that's left is to add little details like rust and perhaps bolts.
I didn't go with bolts at the end of all of this, and settled on just a plain sign with heavy rust. Mmm, rust!
I hope you enjoyed this quick tutorial!
-Posteh