The appearance of an automobile is anything but unimportant for the owner. This applies to the acquisition as well as the keeping. In this context, the avoidance of corrosion is a fundamental part of the user's satisfaction of a company. The body design can be modified to optimize drainage and reduce the risk of corrosion, improving the owner's satisfaction with the purchase of the automobile. During the proof of concept of water management, as part of the process of development, physical prototypes are state of the art. At this point in the development process, every necessary change is expensive and time consuming. Virtual methods are able to support the development in earlier steps and thus reduce costs. The conventional Computational Fluid Dynamics (CFD) methods could not handle the simulation of a full car in the rain or water passage properly due to much higher computation efforts and deviations from the experiments. The necessity of a fine mesh and small time step to calculate the free surfaces properly results in high computation times. Using the Smoothed-Particle Hydrodynamics (SPH) method as a representative of the grid-free CFD, simulations like these are possible even with high detailed models in a much less amount of time than using the Volume of Fluid (VOF) method. This article contains a short introduction to the SPH method, an explanation for the preprocessing of the model, and a detailed presentation of the solution. Finally, the solution gets a comparison to test bench results and a critical discussion.