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I use Daniel Smith's watercolor grounds painted on the surface with two or three coats and then leave it to dry completely. This surface then allows me to paint with watercolor rather than the acrylics that I used to use on the ornaments. The Daniel Smith grounds perform very much like watercolor paper and allow for all the normal watercolor techniques including lifting, glazing, and wet into wet.
Daniel Smith has 5 watercolor grounds - Transparent, Titanium White, Opalescent, Iridescent Gold, and Black. They can be used on almost any surface.
To see the whole process, follow this link: http://www.danielsmithpaint.com/blogs/a-glass-ornament-and-watercolor-ground-demonstration-with-lorraine-watry/