No, you don't have to sandblast an AR lower. The anodized finish is more than rough enough to allow good adhesion. It must be degreased well. I just get a gallon of cheap lacquer thinner and soak in it in a bucket. I bake in the oven in the kitchen.
Do not worry about getting the moly in holes, on threads, etc. It goes on very, very thin and will not fill in serial numbers or other markings.
The cheap hobby airbrush is all you need.
OK, say you spray it and it has runs and looks like crap. Until you bake it (to cure it) the moly will come right off with lacquer thinner. You can always start over.
Man, you will have to be a real klutz to mess this job up.
BTW, the Flat Black Socom will match most uppers, and vise versa. Here is a picture of a DPMS lower and an old A1 upper that I sprayed with Moly Resin. I put them together in this photo just to show the match of color and texture.