I have a video called "Birth of a sewing machine" It's from the Scottish Screen Archive. It looks to be a film from Singer's factories showing how they made machines. It's silent, and about 74 minutes long. I signed a release saying I wouldn't "share" it, but I could probably watch it again and try to describe what I see in the japanning phase.
If memory serves, they set it up a lot like a waterwheel, dunking the machines, then putting them in a furnace of sorts.