We have studied the effect of ultrasonic agitation on the surface of n-type gallium nitride (GaN) layers grown on sapphire that were subjected to a selective photoelectrochemical etch. Solutions of various concentrations of KOH were used along with an ultraviolet lamp to oxidize the exposed GaN surface, resulting in an anisotropic etch. Smooth surfaces with root-mean-square (rms) roughness of similar to 4 nm were obtained for a narrow range of etching conditions. It was found that this window could be extended by using etch conditions which produced "whisker" growth. Subsequent ultrasonic agitation was then used to remove these whiskers and obtain smoother surfaces with the best rms roughness values being similar to 0.9 nm.