I have built a few locks from kits, but not the TOW Bess lock. Final clean -up of all the casting lines I do with jewelers files, then 320 silicon carbide paper/crocus cloth which ever is more readily available,then 600 paper, then 1000 then 1500. When that is all done I put it on the polishing wheel/buffer with red rouge, then ZAM polishing compound.You might not want a highly polished lock though. The interior of the plate I make sure it is flat, by putting a mill file in the vise, and holding the plate flat and drawing it into the cutting teeth of the file(I go back and forth without taking my hand off the plate seems to help me keep the plate in the same position) Then when I'm done and satisfied that its true I'll lay it on a sharpening stone(soft arkansas) and see how flat it is by rubbing it a few times and see where the stone cuts the plate. I don't go real crazy here, But I have spent alot more time on this portion of the work at times than I'll admit. Then I'll polish it. The finer the polished part the less a chance for rust to take a bite into the metal.I try to keep my protruding screws just slightly higher , then dome the ends. You want to try to keep your edges crisp and not rounded, unless you're trying to have a worn appearance.
That's basically what I do. Others may offer another view and options.
.........George F.
P.S. You might want to start your sanding paper with a courser grit, maybe 180-220 would get faster results and get some of the file marks out alittle better.