I love Stephan Martiniere! His work is amazing - try www.goodbrush.com as well - Craig Mullins also produced stunning works!

Anyway, use tyhe 3D base render as a guide - light the general "scene" properly and you might even wanna leave textures out of it if you wanna just play and have fun with where you might end up.

Just like everything else, practice practice practice!

I recently did one myself and I think I'm just gonna keep doing this technique and experimenting and seeing where it takes me.

http://www.renderosity.com/mod/gallery/index.php?image_id=1866866

And attached is the original render out from C4D.

The background general concrete wall is a photo resized and masked, set to Overlay Mode. The rest is versions of painting on layers set to Overlay, Soft Light, Multiply, use of masked Adjustment Layers, etc. Painting in a variety of different methods to get all the texture, grime, highlights and shadows, etc.

I actually first got the idea from Rapheal Lacoste - as he uses this technique extensively, but he goes further and adds characters and stuff not in the render at all because, frankly, he's amazing at it. It never occurred to me to use a base render as a starting ground for a complete painting - now I"m just practising to be able to paint other stuff as well...

We'll see how that goes...

Hope this helps-
-Lew ;-)