Dede - I had great success with 18k plating over sterling (vermeil)
with Cal-Mart Plating Co in Los Angeles.
They were very helpful and explained the normal plating thickness
is in microns and then did a heavy 18k yellow gold plate for me
because I wanted to be sure it would not wear off the brooches I was
Cal-Mart Plating Co
404 W. 7th, Suite 524
Los Angeles, CA 90014.