I took the adjustable front sole off my Jack yesterday to clear any dust from the slideways. Just as I was about to replace I saw that the edge of the throat where the shavings go was black. It was like the resin you see on router bits but I have not machined any resinous wood. I could only remove with some 800 wet and dry and then a polish. don't know if it made any difference, but left there it would not have helped.