For the exterior design of virtual clay model in automotive industry, a novel variable stiffness brush model and haptic decorating method are proposed using a six DOF input device and based on real-time force feedback technology, and the haptic behavior of virtual 3D brush based on variable stiffness is studied in detail. Firstly, the relationship between brush deformation and endured force is examined by employing the bending spring-mass model to construct the 3D brush mechanical model. Then, the collision detection between virtual hairy brush and virtual 3D object is studied based on a collision algorithm of Weighted Average Distance. An effective ball expanding operation is used to compute the smallest bounding sphere of the bent brush and then to determine the projection plane. The 2D footprint produced between the brush and the projection plane is calculated according to the deformation of the brush at a sampling point, and then, the 3D brush footprint can be obtained by projecting the 2D brush footprint onto the 3D object surface. The 3D brush stroke is formed by controlling the exerted force and superimposing 3D brush footprints along the direction of painting. Experiment results show that the adopted method can effectively enhance the reality to users.