Well, I now have an expensive Hoya Digital HD xND4 filter "fused" to a UV filter. Both have thin, essentially smooth rings. Anybody have a good trick to unstick these puppies?

It's an issue long familiar to still photographers.
Try placing a rubber band around the rings of either or both filters. That should give you enough traction to unscrew them without further adieu.

Some photo stores carry a tool specifically designed for this, but Ron's tip about the rubber bands works best. Have done it several times.

I've found that sometimes I just have to keep working it. It's possible that one of the threads is a bit out of round, so grabbing at one angle may make it stick even harder while at another angle it can come off easily.

It can be like opening a jar. Your wife tries to open it and can't. She hands it to you and you can't budge it. Hand it back to her and she unscrews it effortlessly. ;)

Rubber Bands to the rescue! I'll try that when I next have them in my hands.

Thanks!

One of the problems in trying to remove filters stuck like this is that the pressure you exert to actually hold the filter in your fingers can actually 'pinch' the two filters together as the filter slightly distorts.

One tip I can give from 30 years of unsticking these ****** things is to hold the filters flat between your two hands - as if you are praying - and apply gentle pressure as you turn your hands in opposite directions. Works more times than not.

For REALLY stuck ones use a couple of pieces of the black rubber sheet you get in hardware stores that you can put on shelves to stop stuff sliding about, very cheap. More expensie version of the same thing is the rubber pads that you can get for opening stiff jam jar lids, works really well.

All else fails get two of these: Filter Wrench (http://www.srb-griturn.com/filter-wrench-850-p.asp)

Thanks John and everyone else. I guess I'm somewhat comforted to know that I'm not the only person that's run into this difficulty. I guess it's good for one's pride to feel the noob time and again!

I keep a pair of those soft pads for opening jars in my bag. Works every time.

I've used a long zip tie with the ratchet tooth removed as a makeshift filter wrench.

Ive got a similar problem, ive had a uv filter stuck to the front of my canon 17-55 for about a year now! Nothing moves it

What about putting it in the freezer and taking out hours later... Cold VS a warm room and expansion changes? Then use the above tips?

THANKS GUYS! I used the rubber bands and was just able to separate the filters. Now I need to give them a thorough cleaning.

Thanks again!