Acrylic painters can modify the appearance, hardness, flexibility, texture, and other characteristics of the paint surface by using acrylic mediums or simply by adding water. Watercolor and oil painters also use various mediums, but the range of acrylic mediums is much greater. Acrylics have the ability to bond to many different surfaces, and mediums can be used to modify their binding characteristics. Acrylics can be used on paper, canvas and a range of other materials, however their use on engineered woods such as medium-density fiberboard can be problematic because of the porous nature of those surfaces. In these cases it is recommended that the surface first be sealed with an appropriate sealer. Acrylics can be applied in thin layers or washes to create effects that resemble watercolors and other water-based mediums. They can also be used to build thick layers of paint—gel and molding paste are sometimes used to create paintings with relief features. Acrylic paints are also used in hobbies such as train, car, house, and human models. People who make such models use acrylic paint to build facial features on dolls, or raised details on other types of models. Wet acrylic paint is easily removed from paint brushes and skin with water, whereas oil paints require the use of a hydrocarbon.