As far as I know, MacBooks are not painted?
The point being that they painted the sp3 to make it more attractive. Just take a look at the colour underneath the paint, not pretty. And no, macbook is not painted because it is not a magnesium something or other, it is aluminium, a very attractive metal on its own. MS chose their magnesium design for a reason (thermals?) but as a result it required paint to maintain appearances.