Hi!
Not easy to find the cause of this gray edge... could you share your .blend file in order to have a closer look?

Hello. This is a know issue. This is what happens with low poly models. There aren't much you can do. You can try the subdivion script provided in my Blender file to see if it works.

Running the subdivision script definitively made the trick:
http://imgur.com/a/h066Y

If it often gets good results, maybe this technique should be documented to prevent every newbie like me from invoking you...

Thanks for your help!

This has been however, it mainly works with circular or organic parts. For the others with sharp edges and/or square cuts, it has to be fixed manually. This is only an experiment. It would be too good if it would work 100% of the time.