This is a non-traditional method of japanning, but it worked for me.
Before japanning lock the screws in with thread lock(the blue stuff), which locks the handle bolster plate in and the bed plate in.
Later, cut off the excess and lap to flatness. In the photo below, I'm not quite done.
First, clean off any place you want the paint to go. Second, tape any place you don't want the paint to go. Than spray the paint and let dry.