Yes, it will not be much. I saw the video regarding those teflon dams and Dan E said that he has used many things over the years, so I was thinking a credit card maybe? I will also do it before I re-saw the slots so I can clean it if that does not work.
As for the dust, internet advice is fill with dust and add a drop of superglue over it. I can do some light sanding afterward if it is not flush when dry. I a only planning to do 2 larger spots. The rest will be "character marks" if they show at all.
Work looks most excellent....I have a 1970 Hohner 400N that I pulled the frets out of years ago, and I am wondering if I should try to fret it...I don't even have a single old fret to gauge by....