I have been spending what feels like way too much time preparing the vacuum plates.
Overnight in vinegar did remove all of the mill scale.
I do not have a lathe though so I could not do faceing to get a good sealing surface. Instead I used a drill press and a bearing and a hole saw to turn it while i sanded with an angle grinder. The results were quite nice and cleared out most of the divots. I have to go back with finer paper to finish gleaning everything up before cutting the holes for my KF40 flanges.
I now am starting to worry about brazing compounds to allow modest baking of the camber to get a nice base pressure. I am considering using 95/5 tin silver solder do to the steel stainless compatibility. I am worried that the joint won't be strong enough. I have been looking at brazing alloys, but I cannot find anything without zinc.
Anyway I've got plenty of steel left I'll try the solder and see how it holds up.