Basically I did mine like you did, Jim.
Mine are OSHA orange with black letters. I throughly stripped the original paint off of the plates. I removed all of the dings, dents and ripples out of the plates. The hardest part was the edge of the plates, bareing in mind that 1937 plates weren't that straight when they were new. I painted the plate black with Krylon paint. I put 25 coats of black on the fresh metal. This took several days. I define a coat as a fairly quick pass over. After they had dried for severl days, I put one coat of OSHA orange on the total plate, not worrying too much if I didn't cover the letters well. I let the plats dry well, again for several days. I then took 1400 grit wet sand paper and using a sanding block and allot of water, sanded the letters using the block flat. Occassionally I had to work down the edges of a letter to get the full effect of the black paint. Once finished, I rubbed them well with compound. I did not clear coat them, but that can be done also.
This is a pretty long process, but the results are very good.
The idea of power coating the undercoat, or the color of the letters, is good, it saves some time and the powder coat should stand up under the 1400 grit paper and water, but I have never tried it.
After doing a set, I can understand why a pro charges what he might charge. It takes allot of time. The other big secret is that you have to have a decent set of plates to start with.
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