JTR:
The exterior surfaces are "as cast" and there are usually a few mold lines here and there that need removing. The frizzen can be particularly difficult because it is hardened. I use a set of diamond coated needle files for this, but it is still slow going. Quite often the mainspring rubs against the plate in a couple of places and needs a little attention. Also the sear, sear spring and tumbler usually require some attention to obtain a good consentient trigger pull. Be careful with these as a little bit of change can make a big difference. If you do go too far replacement parts are available. Thank you Eli Whitney.
None of these are show stoppers, but they can make a good lock better.