How to –

Prime and stretch a canvas
A Newcastle Arts Centre Guide

Canvas is the traditional painting surface for portable paintings.

However it is not dimensionally stable, it can shrink or stretch with the wetting or drying of the primer,
paint or atmospheric changes. It is important that the Stretcher bars are made of well seasoned timber
and the canvas is strong. Linen canvas has always been chosen for professional work because it is
strong and resistant to rot. Cotton is cheap, less stable and prone to mildew. Canvas must be ‘sized’
to control the absorption of liquid by the fabric.

Careful priming of your canvas is important and this should be done before your canvas is stretched
although it is possible to size & prime a lightly stretched canvas. Priming a canvas requires a lot more
care than a wall or board because canvas is flexible and will crack a brittle primer. It is therefore essential
that the primer remains flexible. Traditionally rabbit skin glue was used as the size because it retained
some flexibility; today a good quality PVA such as Marvin Medium does the job well and is a lot easier to apply.
There is a myth that Acrylic paint can be applied to a raw canvas but this is unwise as the medium is
absorbed by the canvas to leave the paint with a chalky surface that is prone to damage and almost
impossible to clean or restore. After sizing, two thin coats of Acrylic Gesso Primer will create a good surface
for Oil or Acrylics paints. Acrylic primed canvas can be improved for oil painting by the application of a
thin coat of Thixotrophic Oil Primer. A good test for a canvas is to dab a spot of white spirit on the front of
the canvas then check to see if it soaks through to the back, if it does, the priming requires more work.

To stretch your canvas, first assemble the stretcher bars on a flat table or floor.
Make sure that the frame is perfectly square; check this by measuring diagonally, corner to corner.

You will need a staple gun and a pair of canvas pliers.

Fix the frame square by firing a staple across each corner joint.
Cut the canvas to a size at least four inches (100 mm) larger than the frame.
Begin Stapling or tacking from the centre of the outside edge on one of the longest bars
and then stretch to the centre of the opposite bar. Repeat this for the other two sides.
Continue stretching across the frame from side to side. Work out from the centre of each bar
and leave the corners until last. At the Corners neatly fold the canvas on to the back of the frame,
do not cut the canvas to fit the corner. Finish by tacking the spare canvas to the back of the frame.
The canvas should be taught but not too tight because it may shrink as the paint dries.

Stretcher bars are normally supplied with wedges, known as keys, which can be used to wedge open
the joints and thereby tighten the finished painting.

Tip : - Should the finished canvas suffer a dent , don't worry, simply dampen the back of the dented area with a little clean water and leave to dry, it will normally shink flat.

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More 'help' pages - Painting tips - how to use gold leaf - how to Batik

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