Two different jobs get called the same thing, and confusing them is how canvases get ruined.
Stretching blank canvas means preparing a surface to paint on. It is forgiving, and you can wet the back afterwards to tighten it.
Stretching a finished painting means mounting something you have already spent weeks on, where a crease is permanent, a staple in the wrong place is visible forever, and water on the back is a bad idea. Most articles on this subject describe the first job and put the second one in the title.
Both are below, in that order, plus the thing that decides whether you can do it at all: how much blank margin you have.
First, check your margin
This is the step to do before you buy anything.
To wrap a canvas around a stretcher bar you need unpainted canvas equal to the depth of the bar plus about an inch and a half for the staples and the fold. For standard bars an inch and a half deep, that is roughly three inches of blank margin on every side.
Measure it. If you have less than about two inches of clean, unpainted canvas all the way round, you cannot stretch it conventionally, and no amount of pulling will change that. Skip to the section on what to do instead.
A word about our own unframed kits
Since you may be holding one, and this is the honest answer rather than the convenient one.
Our unframed 16x20 canvas is trimmed close to the printed edge, and there is not enough blank margin to stretch it over a frame. Customers have found that out after finishing a painting, which is a bad time to find it out. It is made to be taped or mounted to a board, or handed to a framer who can strip-line it.
If you know in advance that you want a stretched canvas, buy the framed size. It arrives already stretched on a wooden frame, ready to hang the moment it dries, and it needs none of what follows. Our guide to framing a finished paint by number canvas covers the other options, and the sizes are listed on each design's page in the range.
What you need
- Stretcher bars in a matching pair of lengths. Bars an inch and a half deep suit most work; go thicker beyond about 30 inches so the frame does not bow.
- Canvas pliers. The one tool with no good substitute. Ordinary pliers crush the fibers and you cannot get even tension with your fingers.
- A staple gun and staples, 6mm to 10mm.
- A carpenter's square or anything with a reliable right angle.
- Scissors, a pencil, a tape measure.
Allow 30 to 60 minutes. Stretcher bars and pliers come from an art supply shop; the staple gun from a hardware store. We do not sell canvas, bars or stretching tools, so there is nothing for us to point you at here.
Stretching blank canvas
1. Assemble the frame
Lay the bars out as a rectangle and push the mitered corner joints together until they seat. Check both diagonals with a square — if the corners are not true, nothing you do afterwards will be.
Note which face of the bar has the raised lip along its outer edge. That lip must face the canvas. It holds the fabric off the inner part of the frame, so the wood does not print a line through your painting over time. Building the frame the wrong way up is the most common mistake here and it is invisible until it is too late.
2. Cut the canvas
Lay the canvas face down, put the frame on top, and cut about three inches wider than the frame on all four sides for standard bars. Err generous. You can trim afterwards and you cannot add.
3. The first four staples
Fold one long side over the back of the bar and put a single staple in the center. Go to the opposite side, grip with the pliers, pull firmly against the frame and put one staple in that center.
Do the same for the two short sides. You now have four staples and a shallow diamond of tension across the face. That diamond is what you are looking for.
4. Work outwards, always in opposite pairs
Add two staples about two inches either side of each center staple, and keep alternating between opposite sides. Never finish one side before starting another — the tension has to build evenly or the frame twists and the weave goes crooked.
Keep going out towards the corners, leaving the last few inches of each side unstapled.
5. Corners last
Pull the corner canvas tight, fold it the way you would wrap a parcel, and put the fold on the side you would rather not see. Staple through the fold. Then add the last staples along each side.
6. Tighten, if you need to
Turn it over and look across the surface at a low angle. Slack shows as a shadow. Remove the staples in that area, re-pull and re-staple rather than trying to compensate elsewhere.
For raw or unprimed canvas only, a light mist of water on the back will shrink the fibers as it dries and take up small slack. Do this once, lightly, and let it dry fully.
Stretch dry and tighten afterwards. Stretching wet canvas gives you a taut surface that goes slack as it dries, which is the reverse of what you want.
7. Prime, if the canvas is raw
Unprimed canvas needs gesso before paint, or the oil or acrylic soaks into the fibers and eventually rots them. Two thin coats, sanded lightly between. Our guide to what gesso is and when you need it explains why.
Stretching a painting that is already finished
Same sequence, four important differences.
- Work face up on something soft. A folded blanket or a clean towel. Dragging a painted surface across a bare table scratches it.
- Do not wet the back. Water can swell the support unevenly and lift or craze the paint film. If it is slack, re-stretch it properly instead.
- Pull gently and check the front constantly. Acrylic stays flexible for years and will take a moderate stretch. Oil becomes brittle as it ages, and a dry oil painting can crack along the fold line. If the painting is oil and more than a few years old, this is a conservator's job rather than a weekend one.
- Get the image square to the frame before the first staple. Mark the center of each edge of the image and the center of each bar in pencil, and line those up. Once the first four staples are in, the position is set.
If you do not have enough margin
Three real options, in order of how much they cost.
- Mount it to a board. Acid-free foam board or a hardboard panel, with acid-free double-sided tape or archival adhesive around the edges. Cheap, flat, and it can then go into an ordinary frame.
- Float it in a frame. Tape the canvas to a backing board and set it inside a frame with a visible margin around the edge. This suits a trimmed canvas well and looks deliberate rather than like a compromise.
- Have it strip-lined. A framer sews or adheres strips of new canvas to the edges of yours, giving you the margin you lack. It is the proper solution, it costs money, and it is what you would do for a painting you care about.
A canvas panel — canvas already bonded to a rigid board — is worth knowing about for next time. No stretching, nothing to go slack, and it travels well.
Common questions
How much canvas do I need around the frame?
The depth of the stretcher bar plus about an inch and a half, on every side. Three inches for standard bars.
Should the canvas be stretched wet or dry?
Dry. Stretch it dry, then mist the back of raw canvas afterwards if you need to take up slack. Never on a painted surface.
Can I stretch a paint by numbers canvas?
Only if it came with enough blank margin, and most do not, ours included. Check before you start painting rather than after you finish.
My canvas has gone slack after a year. Is it ruined?
No. Stretched canvas relaxes with humidity. Most frames have small wooden keys at the corners that expand the frame slightly when tapped in. Tap gently, a little at each corner, and check as you go.
Why does my canvas have a line printed through it?
The frame was assembled with the beveled lip facing the wrong way, so the fabric is resting on the inner edge of the wood. It has to come off and go back on the other way round.
