The second cast out of the mold (which I unfortunately don't have any pre-cleanup photos of to show that it faired quite a bit better, sorry) had cleanup go pretty smoothly. The little nubs were easy enough to get rid of with a dremel and a coping saw. A few of them left little holes which needed to be filled in, but nothing extreme.
I could have spent a week fixing all of the imperfections but my time table didn't allow for that. The upside is that the piece is supposed to look worn and used, so I can mask some of the minor problems with paint. Here is the cleaned up second cast with primer on it, and the laser cut backplate attached with binder clips while I drill out the corresponding bolt holes. I rotocasted it a little too thick along the top so those ones won't be used.
Now on to paint for the rest of the parts. Again I didn't get too many progress shots here as I was in a serious hurry to get it finished for my deadline. Here are the front barrel parts, the back plate, and the under side heatsink all with their paint.
The paint was varying layers of light, medium, and dark metallic gray on top of the bright silver base coat. I then did a couple of washes in black and brown to make everything look dirtied up. The rust was made using lightly stippled layers of dark purple, then terracotta, then a light orange, and finally a bright yellow.
Also, it's gigantic.