Oh I see.
If you take that example as the original, you duplicate it the white layer.
Therefore you have 2 layers but they are directly on top of each other.
Then use the eraser (25%) and randomly go over the duplicate layer 3 times. That way you have 4 shades of white rivets.
Set the dulpicate layer opacity a little higher than the original.
What happens is the rivets from the original layer will show through where the erased ones are at a lower value. All the others give you the random shades of white of the rivets to simulate more 'wear'.
I usually put the duplicate layer above the orignal, but I don't think it would matter which way.
Skychimp - the one I sent to Cit?, you get the P47 bmp from my post?