A size is made by dissolving 8 level tablespoons of rabbit skin glue in 1 litre of cold water. Soak until all the water has been absorbed by the glue to a porridge consistency (for 1 hour or overnight).
Stir and heat gently in a double boiler (never boil the glue as this weakens it). Open your windows if the smell of hot glue is unpleasant. Always use a clean container and do not make more than you can use in one session. Make a fresh batch for each day’s work.
Apply when still slightly warm with a 5-10 cm stiff brush or sponge onto the canvas. If a smooth surface is required, palette knife the size. When applying the glue size to canvas, do not press so hard as to make the canvas touch the timber support or ‘strike-through’ will result and the canvas will be glued onto the stretcher. To avoid this, use one hand to slightly raise the canvas (from the back) away from the timber support. Do not size directly over areas that your hand is supporting, otherwise strike-through will occur there. Sizing the edges will prevent unraveling.
Keep the canvas flat while wet and under tension. If necessary, lay the canvas face up on the floor (or any flat surface) and weigh down the corners. The canvas will shrink more with the glue sizing than with acrylic gesso. When dry it may be sanded lightly with sandpaper and then resized. The second coat of size should not be sanded. Size is not intended to form a continuous level film.
Glue size can also be applied cold (in a jelly state). Using a spatula, work the glue in well, smoothing the excess off the canvas. This is particularly effective for open weave canvasses. Size acts as a penetrating liquid to fill the pores of the canvas and isolate the fabric from strike-through of subsequent applications of primer and oil paint.
For glue sizing paper, adapt the recipe to make a weaker mix, using 5 to 6 tablespoons of rabbit skin glue to 1 litre of water.