Sheet negs from darkslides? Might it be more practical to fill in on the negs and then retouch on the print, as it's often easier (for me, but I'm a clumsy amateur for a few decades now...) to add density than remove it. The only times I tried knifing it left a noticeable surface-texture mark, even if the density was ok-ish.

Re-reading, it sounds as though it might be too late for neg-work though, sorry.