It would be planed and sanded and if needed, joined with other pieces to obtain the desired size and shape. The wood would be coated with a mixture of animal-skin glues and resin and covered with linen (the mixture and linen combination was known as a "size"); this might be done by a specialist, or in the artists studio. Once the size had dried, layer upon layer of gesso would be applied, each layer sanded down before the next applied, sometimes as many as 15 layers, before a smooth hard surface emerged, not unlike ivory.