I'll have to take everything I learned with this '51D and apply the changes to the 3 '51B skins (Beantown Banshee, Snoot's Sniper, and Bonnie Bea) I'm working on.   I definitely like the bluish tinge to the aluminum, for example.   And I'll have to change their spec, power, and env values too.  
After that its only the N-map, and not sure how I want to handle that.  I think I'll start with a completely minimalist approach.  What do you N-maps look like?  Do you take them down to the rivet level?
My skinning focus right now is to learn how to create absolutely eye-popping bare aluminum skins.  I'm closer now than I was before, but still feel like I can do more.