The perturbation techniques of Friedrichs, Keller and Benney ar used to derive the governing equations of long wavelength surface waves in a liquid film of a fluid which obeys a generalized Norton type consititutive relationship. Motions are assumed to be small so that acceleration terms are negligible. It is shown that the form of the surface wave equation depends verystrongly on the stress-strain-rate relationship at very small strain rates. The result is that dispension is a secondary effect, if the material response is soft at low strain rates, but may be dominant, if it is stiff.