On any plane that I repaint, all of the old japanning is removed by sandblasting with a medium aluminum oxide grit. This also roughened the surface to improve adhesion.
I wash down the plane with Brakleen and let it dry before masking off the areas that are not to be painted. Threaded holes are cleaned with a cotton swab and plugged with a bit of paper towel.
For the actual painting process:
1) Spray on one very light coat of gloss black (DE1613) and let dry for 15 minutes.
2) Spray on a heavier coat of gloss black, letting it "puddle" somewhat on horizontal surfaces. Let dry for 20-30 minutes.
3) Spray on a light coat of semi-gloss black (DE1635) and let dry for two hours. Carefully remove the masking tape.
4) Let dry for two days in a warm room before reassembling.
I have found that the gloss black builds thickness more quickly than the semi-gloss black. It levels the surface and helps it to look like real japanning. The initial light coat acts sort of as a primer, allowing the next coat to be applied heavy.
Here is a picture of the final product. One plane has been painted with the technique I just described, while the other is an untouched original. Can you tell which is which?