Pencil tool smooths un-edited path

I often use the pencil tool to simplify areas of art like complex coastlines in maps. When I draw over an area to smooth over unnecessary detail, I sometimes find that the entire path is simplified. Most times the unedited portion is left as-is, and only the re-drawn area is changed (which is what I'd expect). I may, for example, edit areas of the shoreline in New Jersey, but areas far off-screen like North Carolina may get successively simplified so that parts are unusable. I can't just break it up into smaller paths, because it has to be filled later.

Is there a way avoid the unwanted smoothing effect of path areas I don't redraw?