Simple green is fine. I have always used what ever dish washing detergent that was at hand with good results (Dawn?). An ammonia containg solution might get into more of the grease or oily materials better, strong enough and it will strip off the finish (ammonia dissolves shellac).
I don't finish plates, but you do need to fill and sand any chips and scratches before spraying. Your finished product will only be as good as your prep!
------------------------------
Blaine Hebert RPT
Duarte CA
(626) 795-5170
------------------------------