In the day documents show that the average was 20 hours per machine to hand block sand them smooth.
The process is that you add the MOP first, (we used a drop of paint to stick it down) then japanning was applied, then sand, and sand, and sand, and sand....
My first attempt I used MOP that was too thick and the paint would not set up that thick effectively, when we tried thinner bits it worked out great. I have a basic black Singer 12 parts head that was given to me because it was sand blasted clean resulting in massive pockmarking and the like. I am working on refinishing it using the MOP process. Pictures will happen once done (and working)