Pens are tricky because scratches create ditches with high shoulders on both sides.

Clean the area with Dawn, rinse, dry, then clean the area with rubbing alcohol.
Level the shoulders using your very finest sandpaper from the headlights job.  
Then lay a line of paint down the the scratch.
Next, using a NEW CLEAN (cleaned ahead with rubbing alcohol to remove the oil) single-edge razor blade, gently pull the razor blade along the scratch, leaving just enough paint to fill in the ditch.
Let it dry thoroughly (overnight) and gently smooth it again with your finest sandpaper.  
Clean the area with rubbing alcohol, and apply automotive clear coat from an auto supply store.
Buff when dry.

Your touch-up will probably be darker than the surrounding paint unless the vehicle is white or off-white, but that's just how it is.

For instructions on using spray tough-up paint, there are very good videos at http://www.paintscratch.com