2.
Metal edges
We are now going to highlight the edges of the metal. This will have to
be applied where the metal makes a sharp angle, so dont do it everywhere
! Once you'll have your tools and swatches made, it's going to be really
easy. Let's see how to do so :
2.1 "metal" layer
First, create a layer named "metal" or whatever, on the top
of the "blueprint" layer :
Now create a new swatch named "OFP_metal" or so and use the
following properties (press F6 if you dont have this dialog):
Once it's done, select the Paint Brush tool 
and use these settings (press F4 if you dont have this dialog):
Click on the "Presets:" icon and save your preset as "OFP_Metal"
or so.
Now use these settings to paint all the sharp edges of your textures.
simply drag and drop your mouse to apply it. make it rounded on the edges.
Pass twice by the same way if necessary. Apply it this way (i use the
blue arrow to show the way to paint it... dont paint any blue arrow ;-)):
In the end you should have put it as shown by the blue bondaries :
The other bondaries will be coverd with some rust or dirt later on, so
dont paint it now.
2.2 "metal 2" layer
Create a new layer named "metal 2" or so and lower the transparency
of "metal" to 30% :
Now create a new swatch named "OFP_metal2" or so and use the
following properties (press F6 if you dont have this dialog):
Create another preset for your paint brush tool and use the following
settings (and name it "OFP_metal2") :
Select the paint brush, the "metal2" swatch and preset, and
then apply it in the exact same way as the "metal" tool, and
at the exact same location. Lower the layer visibility to 60%, and if
you hide the "blueprint" layer :
You should obtain something like this :
You can download it here : sample
3.zip
We are now going to put some rust on the other edges.
Note : If you use a very dark colour, as dark green,
dark gray or black, then i'll recommand you lower the transparency even
more... But wait first for all the step to be complete. Try to experiment
a bit and have the best effect on your vehicle.