Unlike solid parts, the deformation caused by a contact force during robotic polishing of sheet metal parts has become an issue. In this paper, a path planning method is purposed to resolve this issue. This method includes three steps. The first step is to apply the Hertz theory to compute the contact areas between the tool head and the free-form surface of a sheet metal part. The second step is to apply the finite element method to compute the deformation under a contact force. The third step is to reconstruct the deformed free-form surface and modify the contact areas accordingly. The underlying problem is dynamic because the deformed surface shape changes as the tool head moves along a tool path. Based on the proposed method, an optimal path can be determined to achieve full coverage of the entire surface without over or under polishing.