I have scraped gunk off ropes, especially dry ones where a section barely has the crowns visible. Just a small screwdriver down the valley seems to work alright
I have heard of pouring powdered borax on the ropes and wire brushing. Seems a bit sketchy to me, I remember hearing to not use a wire brush on ropes, and an acid doesn't sound like it promotes rope life.
Also tried using rope oil and carpet. Didn't seem to do that well. Maybe as a last step to clean them
