No protective paper? What a bummer.
Ponoko says the following:
"Most of the acrylic sheets we use come with protective paper on both sides. It’s possible for us to leave this paper on when making your design, which we tend to do where it will not interfere with your engraving. The main downside to this is needing to peel paper off both sides of the acrylic, which can be time consuming and tricky if your design is intricate."
Maybe Formulor removes it, because they think it's customer-friendly.
I can confirm Altitude's method, works too with acetone and fresh paint. Acetone is not corrosive on the black and white Sammich cases and doesn't leave a stain.
But when you use black paint on the white acrylic panel, some black particles are left. You can see that with the yellow particles on Altitude's panel. Doesn't look that clean to me.