Experiments were performed in capillary and slit rheometers to determine the effects of the presence of fluoropolymers at the polymer-wall interface on wall slip and extrudate distortion of a molten high density polyethylene. Two fluoropolymers were used to modify the interface. It was found that one of these decreased slip compared to the clean interface while the other increased slip. However, both were found to improve extrudate appearance, thus indicating that both adhesion promoters and slip promoters may improve extrudate appearance. The mechanism underlying this rather surprising conclusion is also discussed.