I have visited several stretches of sand dunes over the years (southern Colorado, Death Valley, Oregon). I seem to have trouble getting good negatives when photographing them. I am careful about calculating the exposure (I use a Pentax 1 deg. spot meter) and I develop the negatives carefully and at times consisten with my Zone System testing. Still, they always seem to come out thin routinely.

Is there something about the sand that (like snow) throws off the meter?
Would the use of some filter help out? (I recall reading that E. Weston asked Adams about this and Ansel recommended using an orange filter....).

Are there any experienced B&W sand dune photographers out there that can offer some advice? I will probably be going back to Death Valley after the summer.