I have a nice Singh-Ray Warming Polarizer Filter that screws onto my camera lenses, and I have some Lee Filter Graduated Neutral Density Filters. But, I often want to use a Polarizer and a Graduated Neutral Density Filter at the same time. Here's how I solved this problem.
There are two ways to do this. One is to screw the Polarizing Filter onto the lens then screw the Lee Filter Adapter onto the Polarizing Filter, and then attach the Lee Filter Holder. By stacking the two filters, it’s difficult but almost possible to turn the Polarizing Filter to get it to work. The second problem with this system is that, with a wide-angle lens, there is an incredibly large amount of vignetting in the corners of the image.
Another approach (and the one I'm using now) is to put the Lee Graduated Filter on first with the standard Lee Foundation Kit (filter holder) with the Lee Filter Adapter (77 mm for my Canon Lenses). Then attach the Lee Filters 105 mm Accessory Front Thread Adapter (four screws hold it on the Foundation Kit, some assembly required). Then the Lee Filters 105 mm Landscape Circular Polarizer Filter (this is thinner then the regular 105 mm Circular Polarizing Filter) screws into the Accessory Front Tread Adapter. Slightly more expensive, Yes! But, no vignetting, and it’s easy to use the Graduated Neutral Density Filters and turn the Polarizing Filter!
Here’s what it looks like, when I was in the Palouse on Steptoe Butte on the 70 to 200 mm zoom.