Impasto is the technique of adding texture to a painting by applying, with brush or artist knife, thick daubs of paint to the canvas surface. This technique uses up a lot of paint, which is expensive. Artists have other ways of building texture in a painting. Usually, this involves applying a gel medium or modeling paste to the canvas before starting to paint, but a few additional options exist, too.
Acrylic gel medium is a thick, smooth substance, which appears milky white in the jar. Apply to the blank pretreated canvas with a wide, flat brush or artist's knife. Leave to dry for several hours, preferably overnight. It dries clear, and that is how you know the medium is ready to paint over. Paint over the medium and canvas with acrylic. Artists also mix gel medium with acrylic paint to make it thicker. Some gel mediums are also designed for use under watercolor paint.
Acrylic modeling paste is similar to the gel medium. It is thicker than the gel, about as thick as stiff whipped egg whites. Apply it to the pretreated canvas or hard surfaces such as wood, Masonite and medium density fiberboard (MDF) with an artist's knife. Leave the paste to dry overnight; it remains white and opaque when dry. When the paste is ready to paint over it feels firm to the touch and gives slightly when pressed. Don't apply too thickly or you run the risk of it developing cracks when it dries.
Gesso is canvas primer. It's applied to an untreated canvas and coats the canvas cloth, which keeps the cloth from absorbing and bleeding the acrylic paint through to the other side of the canvas. Gesso is a mix of plaster of paris, gypsum or chalk mixed with glue. It's thick, white and remains white after drying. Gesso applied thickly works well as texture. Simply pretreat the canvas or buy a pretreated canvas and apply the gesso in thick layers with a wide, flat brush or artist's knife.
Sand and Pumice
Subtle, gritty texture is achieved on a canvas or board with sand and ground-up pumice. Paint the surface with acrylic paint, gesso, gel medium, modeling paste or even glue. Sprinkle the sand or stone onto the wet surface and leave to dry. Gently tap off excess sand or stone off the surface.Paint over with acrylic. The sand or stone can also be mixed into wet paint and applied to the canvas directly.
Artists also add texture to paintings with mixed media applications. Scrunched-up plastic bags, crumpled paper, different pieces of cloth and even metal sheets and wires may be glued to the surface. White glue is good enough for paper, cloth and plastic. Wood and metal need wood glue, super glue or may even be woven through a canvas.