Re: Cleaning 200+ year old wood floors.
Personally I think the floors look great. If the nails bother you you or you want to refinish you can set them deeper with a nail set and fill. I would treat the gaps the old fashion way by tapping rope in the gaps to fill. That was adapted from work on old ships. You might try cleaning an area with alcohol, if it is shellac it will re-liquefy. It may be varnish in which case you can buff with steel wool or fine sand paper and re-coat.
Be sure you live your life, because you are a long time dead.-Scottish Proverb