I blue at least once a week. Your best bet if there is rust pitting, is to rough sand blast with some aluminum oxide first to remove all the rust. This will be kinda ruff like them new 870's. If the surface feels to ruff, you should go back over it with a fine glass bead blasting. Thats what I normally do.
If its really pitted deep, there is nothing you can do other then removing alot of metal....which may not be a good idea.
If the rust pits are'nt to bad, just a fine glass bead blast will be good enough.
I like to also high polish worthy guns and then glass bead it with some extra fine bead. It gives it a black silky look that almost shines but just enough matte to be non-glare. really looks good to me