Off-line programming (OLP) techniques can be used to reduce programming time and to be able to better control process parameters, such as spray distance and nozzle orientation. This paper presents a simulation method to predict coating thickness build-up on complex geometries. Discretization in time and space is done by using a finite difference model. The model starts with an empirical deposit rate function, which calculates the thickness on a flat surface normal to the spray cone. This model is integrated into a commercial robot simulation program (IGRIP) where discretization of a free form surface into planar polygons is carried out. The thickness is then calculated by stepping the process in increments of time. A robot trajectory is calculated, maintaining spraying distance and normal orientation to the surface. This trajectory is optimized numerically giving a collision free path and a uniform coating thickness. Devices are then programmed, the programs are translated and finally downloaded to the robot controller.