After struggling with digital UV varnish for a couple of months, just a few tips and suggestions:
1) Use UV coating specifically for toner fuser output. You will never, ever get UV coating for offset ink machines to stick on anything that comes out of your 6500, no matter what kind of paper you use.
2) Paper choice still does affect things - What you want is an uncoated 10-12point stock, these papers are the best for "holding" the UV.
3) You want a slow run under the UV, rush it and like in Seinfeld, no soup for you.
We tried a literal slew of ways to get UV on, including varnishing it on a varnish machine, AQ glossing on an offset press then UVing, spraycoating it with varnish beforehand.. but the 3 tips I mentioned will probably help you the most, in order of importance. #1 is the most important, no matter what machine you are using, what paper you have, and how fast you run the UV, you won't get any results (that you'd want to give to a client) without the proper UV coating.