NIce work, but it's kind of messy. I recommend that next time you try to cut more cleanly, and then sand faces flat by taping sand paper to a hard, flat surface (i.e. the floor), and sanding very carefully. Then use a ruler and an exacto knife to cut the stickers.
Keep working on it!
Yep. I have one mod in my shelf, which is a rhombic dodecahedron from a 3x3x3 and I sanded the sides in this way. Just attach sand paper to the ground or floor. You should also tape the puzzle together except the side you want to sand of course, and than just push the puzzle back and forth.
I am not a pro in building but it should make the side much smoother.