If you are looking to get very involved to do detail work, you can pull it apart, file everything to look the way you wish, then give it a quick sand blasting (NOT bead blasting).
Degrease, preheat, then spray it with Norrells and you will end up with a great looking BUIS. From the pictures, that one looks pretty good already, and I don't think you will have that much work to do.
The other thing you can do, which may be a good idea if you didn't know that there are ballbearings inside that rear sight assembly, or if you are uncomfortable with pulling it apart, is to file , then rough the cut area up with sandpaper. The area that is already anodized doesn't need to be sanded. If you have access to blasting equipment, you could tape off your cut area and give it a quick blasting also.
If I were doing it, I would file and sand it into shape, then I would spray the entire assembly already together. I've pulled carry handles apart, and spent a lot of extra time dealing with each individual part, and I've sprayed them assembled, and when you are done, you can't tell the difference.