I think these rugged and deeply patinated items are indeed the most fascinating of historic arms, as they are the workhorses which likely saw use and probably were there in important times and events. With that I would implore you to use all restraint in the cleaning of this sword, and a light but as required use of WD40 to stabilize any active rust or corrosion. That dark patina is history quite literally embodied, and deserves to remain as the well seasoned time this sword has seen. A light coat of gun oil will act to preserve it, and keep its pride and respect.
Again, as Mark and Cutlass have noted, this sword does seem 'colonial' and of the 1780 period, but it is hard to say whether Spanish or American. It should be remembered that countless arms of Spains colonies were present throughout the Americas including our colonial regions, and were actually well represented among many international examples during the Revolution.
I think the affirmation of the hexagonal section on this blade more firmly sets this in Spanish provenance, and again the scalloped shellguard was much favored in Spanish context. The structure of the guard resembles espada anchas, and as discussed over years, many espadas with either scalloped or striated shellguards seem to have been popular in Spanish colonies in South and Central America as well as the Caribbean.
It is interesting to note that these straight DE blades typically regarded as Spanish 'dragoon' types of around mid 18th century (many earlier, see Brinckerhoff & Chamberlain) are known even to occur in the familiar Moroccan sa'if commonly termed 'nimcha' . In maritime settings, the 'Spanish Main' reached into these regions in that trade sphere as well.
Personally, and I know Mark will agree, these worn and patinated examples are my favorites, and as the sentinels of history will often share many of thier secrets, acting as your guide into history. Happy adventures !
LOL! Crossed posts Nando, gettin' a little slow these days, but I'll usually get there sooner or later