Painting is a very common appearance on the surface of a real-world object. On the other hand, the bidirectional texture function (BTF) [Dana et al. 1999] is also commonly used to present realistic appearance of the surface of real-world materials. This motivates us to simulate paints on BTF data since it will further expand the applications and usability. The BTF provides a realistic representation for real-world materials; however, the representation does not allow users to control or edit the BTF data in a simple and effective way. As BTFs are indeed image data, they do not contain any physical information of the material, such as geometry or reflectance. There is no straightforward way to model the physical and chemical process of painting a BTF material. Although some approaches have been proposed to edit BTF data, they did not address the painting problem. 2013 Copyright held by the Owner/Author.