Using polymer processing aids (PPA) based on fluoropolymers not only reduces die pressure, but also postpones extrusion defects for linear polyolefins. The purpose of this work is to investigate the mechanisms explaining PPA efficiency. An experimental set-up was developed to study the PPA actions at different scales, based on a slit die with transparent walls and removable steel inserts, in line with a laboratory extruder. Original results are presented, correlating the evolution of die pressure, surface defects, velocity profiles measured by Laser Doppler Velocimetry and PPA die coating analyzed by Scanning Electronic Microscopy. During extrusion, PPA droplets and stretched aggregates appear first randomly on the whole die surface. Then, coating morphology progressively changes, leading to a network of interconnected and heterogeneous PPA streaks. Quantification of surface deposition shows that the PPA concentration grows towards the die exit, in concordance with spatially evolutionary wall slip. Surface defects elimination is correlated to the presence of discontinuous deposits at the die exit. (C) 2010 Elsevier B.V. All rights reserved.