This instructable was submitted by Xerocraft Hackerspace in Tucson for the Instructables Sponsorship Program.
QR codes are 2-dimensional barcodes that can store quite a bit of information, like website addresses and SMS messages. Best of all they can be easily scanned by any smartphone with one of dozens of free QR scanning apps.
I've seen several methods for creating spray paint stencils of QR codes that require CAD software and CNC routers. I've come up with a much simpler method you can do in a couple of hours.
Step 1: Assemble Your Parts and Tools
1. Fence wire (or chicken wire) with square holes.
- You can find this kind of fence wire at most any hardware store. It's sold cheap and by the foot. The size of the square holes range anywhere from 1/16" up to 1/2", allowing you to make many different QR code stencils in many different sizes. In this instructable I'm using 1/4" fencing.
2. Masking tape (the wider the better)
3. Xacto knife (#11 blades recommended)
3. Pliers for cutting wire (the smaller the better)
4. Two sharpies of different colors.
5. Black spray paint
Step 2: Create Your QR Code
You can use a browser-based website to create you QR code. I used Digital Inspiration's QR Code Generator to create a code to our Hackerspace's Facebook page.
You can also use RACO Industries' QR Code Generator to add a level of error correction if you're worried about the final piece being readable. I did not have this problem though, even with the crappy stencil I spray painted.
Now count the number of pixels along the perimeter of the QR code so you know what size to cut the fencing to later on. Since this is just a simple URL code it is 25px by 25px.
Step 3: Cut the Fencing
Choose a corner of the fence that you want to use. Count the number of squares you'll need for the QR code. In this case, it's 25 squares. But bear in mind that a QR code requires a white border around it in order for it to be readable. For that reason I recommend you add a 1 or 2 square border around the code.
In this instructable I will be adding a 2 square border on each side. That means instead of cutting out a 25px by 25px piece, I will cut a 29px by 29px piece of fence.
Remember: measure twice, cut once!
When you make your cuts, make them close to the wire you're cutting parallel to (what will eventually become the outer line of the stencil template) in order to get rid of any small nubs of cut metal that can scratch you. If necessary, you can file those sharp pieces down.
Once you have your square cut, it's a good time to flatten the piece out before you go any farther.
Step 4: Add the Mask
Lay several strips of masking tape over the fence. One layer should be fine, but two would make the whole thing sturdier (though harder to cut through). Have the sides of each line of tape overlap slightly. After you put a strip down press the tape firmly against the fence.
You can cut the end pieces off or fold them over the fence to cover any sharp pieces.
Step 5: Mark the Squares to Cut Out
The masking tape-covered squares you cut out of the fence will be the black pixels when you use black spray paint. So now you need to mark which ones need to be cut away.
Before you start marking pixels, I recommend you use a sharpie of a different color to mark that 2-pixel border added earlier. You don't want to start marking pixels only to realize half way through that you forgot about that border.
Next, mark the squares that will be cut out. Pull up the QR code you created earlier on your monitor to use as your guide.
Step 6: Cut Out the Template
Take your Xacto knife and cut out all the black squares. Squares that are grouped together can be cut out in larger chunks, simplifying the process.
Cut the squares by placing the tip of the blade just inside of the wire that borders the square you want to cut out. Cut towards the other end while lightly pressing against that wire to use it as a guide and make a cleaner cut. A light source under the template is recommended so you can see the wires of the fence through the tape.
Next, cut over the wires that run in between each square (see second picture). Once those little cuts are made, the chunk of tape should pull right off (third picture).
If you make a mistake, just cover the hole with a small piece of masking tape and continue.
It took me about an hour to cut out the black pixels. Remember, you don't have to cut out every square individually. Instead, cut out squares that are grouped together as larger chunks.
Step 7: Use Your Stencil
Your stencil is now ready. If you didn't flatten the fencing before or it's gone crooked again, flatten it out before use to make cleaner stencil sprays.
Place the template onto the painting surface and make it as flat as possible. Place tape on the border to adhere to the painting surface if necessary.
Instead of one thick coat, try several light coats to maintain a clean stencil.
Even flawed sprays like mine (forth picture) still work with my smartphone.
An idea I had but didn't try was to use adhesive spray on the underside of the template, stick it to the painting surface, spray paint it, then peel the template off.