Emerging artists... this one's for you!
If you have yet to learn how to stretch your own canvas, try it! As an artist, your work isn't just about the subject matter, it's also about the materials you chose to use. There are numerous options out there for supports but I'm going to show you the go to for a painter... canvas.
This canvas will be an 18"x 24".
To create it, you'll need a really good carpenter to make your stretcher bar frame. Just kidding! While cutting the stretcher bars themselves is an aspiration of mine and many artists, you can purchase different types of pre cut stretcher bars from art stores as well as online.
Check out the links provided to easily order products like the ones you see pictured.
You will need:
2
18 inch stretcher bars
2
24 inch stretcher bars
To make the frame, join the 18 inch stretcher bar and the 24 inch stretcher bar together by inserting the "tongue and groove" ends at a 90 degree angle with the rounded edge on what will be the outside of your frame as shown.
Add the remaining 18 inch stretcher bar to the open end of the 24 inch stretcher bar in the same way before adding the remaining 24 inch stretcher bar, which completes your rectangular frame.
If yours doesn't look like a rectangle but it all fits together... perfect! It should lay flat though. If it doesn't, there may be a flaw in a stretcher bar or you may just need to flip one of the stretcher bars and put it back together. When it's laying flat, make sure each interior corner of your frame is 90 degrees. The easiest way to do so is to use the
carpenter's speed square shown. You can use a door frame or the corner of a table too but only if you know it's a 90 degree angle. If you have to adjust, check the other angles again... they affect each other.
When you have 4 90 degree angles, use a
craft staple gun to secure each corner as shown. You can now move the frame around without losing your rectangular shape.
You don't need an expensive or heavy duty staple gun to do this... it's not a box that heavy furniture parts can come in. With that said... for large canvas sizes, using thicker stretcher bars, creating a heavier frame may make you want to use a
heavier duty craft staple gun, wood putty, and braces.
For this frame, all we need to make the frame extra secure is 2 more staples in each corner as shown. You will end up with 3 evenly spaced staples in each corner, holding it together.
I am going to show an
unprimed duck canvas being stretched, however you
can do the same thing with
primed duck canvas. The difference being
that the last step, applying gesso, will be optional for those using
primed canvas.
Unroll your canvas so that the side you plan to paint on is touching the table and you are looking at what will be the back. Place your frame on top. The staples can face up or down, however I have found the staples are more secure if laid as shown. You want to lift your canvas around the stretcher bar to see how much needs to be cut in order to fold over the wood without completely covering the wood. I found this stretcher bar needed to be cut 1 1/2 inches beyond the frame as marked.
With the frame centered on the cut canvas, there should be about 1 1/2 inches of canvas we can stretch surrounding it. I recommend using
canvas pliers to bring equal tension when you are stretching the canvas.
Gently lift the canvas at the center of one side securing it to the wood with a staple. Now you can gently tug and lift the canvas on the opposite side of the frame. To do this, the canvas pliers handle should be moving toward the center of the canvas. Secure it with a staple.
Repeat this until your canvas appears as shown.
Now you are ready to use your hand or the pliers to gently pull the canvas away from the center staple but toward the inside of your frame. Secure it with a staple. This is key to ending up with a tight, wrinkle free canvas. If the number of wrinkles you see is growing, you'll have to remove staples and try again. Removing staples weakens and can damage your canvas, so again, pull the canvas away from the center staple and towards the inside of your frame. Always work from the center of each side.
Here's a tip: After putting a staple on each side of the center
staple, with each staple about 2 or 3 inches apart, move to the opposite
side of the frame and repeat until you have 4 sides that contain 3
staples each.
Make it look professional by keeping even space between your staples and making your staples go in the wood in the same direction.
Your canvas will look something like this and shouldn't have any wrinkles at this point!
Okay you may want to take a break, but I say pat yourself on the back and get ready for the tricky part... wrapping the corners!
It's really not difficult, just pay attention.
Pick a corner and pull the canvas, with your left hand, toward the center. Hold it down on the wood as shown. You will end up with two pockets on each side.
With your right hand, insert your index finger in the top pocket as close to the corner as possible. Now pinch the canvas with your right thumb pulling up toward the sky and laying it down along the stretcher bar under your left hand. Hold it there.
The goal is to create a very flat fold on each side of the corner.
Secure that fold with a staple, making sure that your finger is not stapled to frame as well! Also, remember to avoid the split that still exists where the two stretcher bars come together.
Now take your left index finger and try to get as close to the corner of the wood as possible before pinching the canvas with your thumb and pulling toward the sky.
Then lay that down along the stretcher bar to your right. Hold it there.
Add two staples as shown and you did it!
Repeat this for the remaining three corners and you will have just stretched your first canvas!
This last step is optional for those using primed canvas and not optional for
those using unprimed canvas. Your canvas, if unprimed, will be looser
at this point than if primed. Paint will adhere better, the canvas
support won't break down as fast, and you can apply more layers on top
of the canvas if you apply a layer of gesso. It's a primer. I recommend
Pro Art gesso as a good product for beginners.
Make sure to apply a smooth, even layer with a
flat bristle brush. Two coats will probably be needed.
After applying the gesso, you'll notice that it tightens the canvas even more, which is why some artists prefer stretching unprimed canvas over primed, which is stiffer initially and therefore harder to stretch and stay tight on the frame. Either way, you now have the knowledge and access to the tools you need to stretch a canvas and start painting!
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For this frame, all we need to make the frame extra secure is 2 more staples in each corner as shown. You will end up with 3 evenly spaced staples in each corner, holding it together. 
Unroll your canvas so that the side you plan to paint on is touching the table and you are looking at what will be the back. Place your frame on top. The staples can face up or down, however I have found the staples are more secure if laid as shown. You want to lift your canvas around the stretcher bar to see how much needs to be cut in order to fold over the wood without completely covering the wood. I found this stretcher bar needed to be cut 1 1/2 inches beyond the frame as marked. 
With the frame centered on the cut canvas, there should be about 1 1/2 inches of canvas we can stretch surrounding it. I recommend using canvas pliers to bring equal tension when you are stretching the canvas.
Now you are ready to use your hand or the pliers to gently pull the canvas away from the center staple but toward the inside of your frame. Secure it with a staple. This is key to ending up with a tight, wrinkle free canvas. If the number of wrinkles you see is growing, you'll have to remove staples and try again. Removing staples weakens and can damage your canvas, so again, pull the canvas away from the center staple and towards the inside of your frame. Always work from the center of each side. 
Here's a tip: After putting a staple on each side of the center
staple, with each staple about 2 or 3 inches apart, move to the opposite
side of the frame and repeat until you have 4 sides that contain 3
staples each. 
With your right hand, insert your index finger in the top pocket as close to the corner as possible. Now pinch the canvas with your right thumb pulling up toward the sky and laying it down along the stretcher bar under your left hand. Hold it there. 
Now take your left index finger and try to get as close to the corner of the wood as possible before pinching the canvas with your thumb and pulling toward the sky.
