In the extrusion of linear low-density polyethylene (PE), fluoropolymer processing additives (PPAs) are used to eliminate the surface defect known as "sharkskin" by coating the die wall and inducing slip at the PPA/PE interface. We describe an in situ optical method for measurement of the coating thickness by exploiting the phenomenon of frustrated total internal reflection. By correlating the optical and pressure measurements, extrudate appearance and auxiliary experiments, we can elucidate the kinetics of the coating process. The PPA droplets first adsorb in the entrance region of the die and migrate under shear stress towards the capillary exit where they act to suppress sharkskin. We find that a uniform coating in the range of 25-60 nm is sufficient for sharkskin elimination. The steady state coating thickness near the exit ranges from 200 to 400 nm depending on the shear rate and concentration. We develop a mass balance model for calculation of the steady state coating thickness which compares well with our experimental data. (C) 2003 The Society of Rheology.