We find electron-interface scattering potentials for surface electrons floating over the surface of a helium film. The Hamiltonian for the electron scattering by the interface roughness of the solid substrate is obtained for substrates with an arbitrary dielectric constant. Our results for the ripplon-scattering Hamiltonian reproduce previous results obtained by using the electron-atom polarization model. However, this model gives correct results only for the electron-roughness scattering for substrates with a small atomic polarizability.