A ground or primer is the background surface on which you paint. It is usually a coating such as a gesso primer, which physically separates your painting from the support. It is the foundation of a painting, applied onto the raw canvas, paper, or other support. It helps to seal and protect the support, for example keeping linseed oil from seeping into the support when oil painting, and it also provides a better base surface for subsequent layers of paint.
A ground is different from size, which seals the pores between the fibers of the support and is a necessary first step before painting with oils and applying a ground layer.
There are different types of grounds depending on the surface you like to work on, from smooth to textured. Grounds traditionally have some tooth to make the paint adhere better. Grounds should also be chosen depending on the support that you’re working on. Canvas expands and contracts so it requires a flexible ground.
Before the 1950s, all gesso was made of animal glue. Since the mid-1950s, when Liquitex acrylic paint company created the first water-based acrylic primer or gesso, acrylic gesso has replaced animal glues and can be used under both acrylics and oil paints. Many artists use acrylic gesso as it provides a flexible, durable, and adhesive paint surface.
Acrylic gesso can be used as a ground for both acrylic painting and oil painting, although when used with oil paint on canvas, it should be used thinly since it is more flexible than oil and may cause the paint to eventually crack. Acrylic gesso is ideal for acrylic paints and can also be used when oil painting on a board or on canvas adhered to a rigid support.
You can also use an oil-based ground when painting in oil, such as Gamblin Oil Painting Ground (Buy from Amazon), which is a non-toxic alternative to traditional lead oil grounds and is flexible and relatively fast drying. Also, due to the higher percentage of pigment to binder ration than acrylic gesso, only two coats of Gamblin Oil Ground are recommended rather than the four coats of acrylic gesso that are suggested.
Remember that you can paint with oil paint over an acrylic gesso but you can not paint with acrylic over an oil-based ground.
A ground can be any color, although white is the most common. However, it can be difficult to get an accurate reading of values and colors on a bright white canvas. Since, due to simultaneous contrast, most colors appear darker on a white surface than they do when adjacent to other colors, many artists prefer to tone their canvases before painting. To create a colored ground, color can be added to the primer or a layer of color applied over the priming.
An absorbent ground is one that pulls in or absorbs a paint, rather than letting it sit on the surface. Golden Absorbent Ground is an acrylic ground that creates a porous paper-like surface when applied as a layer over acrylic gesso, enabling staining techniques and the use of water-based media such as watercolor, and pen and ink. It is lightfast, permanent, and flexible.