In painting processes, highly viscous and non-Newtonian fluids are often used. An example is the application of PVC materials for seam sealing over the joined edges to close small openings that are difficult to shut with other materials. The parameters of the free jet and the material properties are of great importance for the quality of the sealing. The work is focused on the seam formation process with the thin jet spraying using ANSYS-Fluent as a CFD program for numerical simulations. The objective of seam sealing simulations is to develop a model in the most efficient form considering all the relevant application parameters (dose, nozzle design, operating parameters, application geometry) and material properties for process optimization. Regarding the physical process, the flow has a high viscosity and presents a rheological behaviour under given production conditions. These rheological models for the non-Newtonian behaviour of the coating materials are implemented by the so-called "User-Defined functions" and transport equations. The Volume of Fluid (VOF) method in ANSYS-Fluent is used, in which a two-phase flow with two immiscible fluids (air and PVC) and their interface are simulated. The boundary phase of both Newtonian and non-Newtonian fluids (Herschel-Bulkley-law and Thixotropic model) are simulated and compared. Different geometries, in order to represent the automotive bodywork geometry and a round nozzle are generated with the CDF program ANSYS-Workbench. The investigation of the grid sensitivity is studied by the generation of various meshes and refinements with the Adaptation method in ANSYS-Fluent. The results of all these simulations with different parameters and viscosity models are compared with experimental investigations to validate them in order to find the optimized model, which fits to the thin jet spraying in the seam sealing process. Physical phenomena that may occur during the seam sealing process, like the Buckling effect, and the seam ending have been performed.