These manufacturers are both in the UK, but they probably will ship to you:
http://www.fhlambert.com/processes/clear_coat.htm#.WUzvbYpLf9B
The first one does electrophoretic lacquering, a word I forgot: this is depositing lacquer like plating. It never comes off.
Given the sheer size of the Mexican jewellery industry I would be very surprised if you can’t find similar Mexican
products. Here’s a hint: use Google Translate and that should solve a lot of your problems.