For my last 3 iPads I've installed a Skinomi skin on the back. I leave the glass clean cause I don't like the screen protectors unless they are glass. But the prices for skins for the 12.9 inch Pro are pretty pricey. How easy is it to scratch the aluminum? I currently only have a Smart Cover on it as I hate how thick cases make the iPad, so I'm debating if I should get a skin. Edit:It was supposed to say "Air/Pro" back in the title.