Spray Chalk Outline
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The stencil in this photo is designed for spray-chalking pavement. It is long enough to put one's feet on its ends to hold it down while spraying, if you don't mind overspray on your shoes..
If making many symbols in one day, eventually the residue on the stencil will become sloppy and contaminate the underside creating a mess on the pavement. Something like paper towels will be necessary to dry the stencil from time to time. Wiping it off will solve the problem and it can be put right back into use.
Another issue is how to carry the wet stencil without blotting it dry after every spray. Stapling furring strips to each end would create a handle on one end while weighing down the other to reduce its tendency to flop around depositing wet chalk on the person carrying it.
Avoid vertical surfaces
Rain usually cannot remove chalk from them
I could not find spray chalk Wal-Mart or Michaels or Sams. And the building supply companies like Lowes and Home Depot had only pavement marking sprays that were too permanent. However, I did find something at Hobby Lobby.
Air Chalk.
Recommended. I tested a brand of spray chalk called "Air
Chalk" distributed by Goodmark. It's the correct light, bright blue for this
symbol. It sprayed well and could be removed with a hose. A 3
ounce can is small and cost $1.99 at Hobby Lobby,
but it is pocketsize, which is handy. The top image at right is how it looks on
concrete.
The importer says Wal-Mart carries it,
though I could not find it there. Perhaps it was someplace I didn't look.
Wal-Mart's web site produces no evidence of it either. All I could find there
was a toy Crayola Sidewalk Paint Sprayer. Who knows? Maybe that would work.
At Hobby Lobby Air Chalk wasn't in the places where the first three clerks looked. A
manager found it in the seasonal section. If you find it at Wal-Mart, tell me
where.
Inverted Marking Chalk
.
Not recommended. One company for this that I
found on the web is Cornerstone.
The cans are held upside down, as are all pavement marking products except Air
Chalk. They sell
individual 15 ounce cans for $4.25 each (shipping a single can is $4.25).
The instructions say not to apply it if it is going to rain within four hours.
It had rained half an hour before I applied it to wet pavement. Within an hour
it rained again. It was unaffected. Four hours after applying it, when things
had dried out, I sprayed it hard and long with a hose. The image became slightly fainter
but did not wash away. This product is too durable and turns your message into
graffiti that is more likely to turn people against you than to persuade them of
your message.
Also, this product has such a strong blast that overspray is a problem. Stencils
would have to be masked at least a foot in each direction, making it 3 ft. by 3
ft., a large cumbersome stencil with which to deal.
Air Chalk is a better solution if you can find it.
Rust-Oleum
Spray Chalk. Not recommended. Kuzsports
sells this in 17 ounce cans for $6.00 ($9.27 shipping and handling for a single
can that can take 5 days to arrive) or a case of 12 cans for $62. One can sprays
a line 500 feet long (which I'm estimating will spray 150 templates). It has
overspray problems, but its main problem is that it is paint. When trying to
remove it the hardest spray from a hose had no effect. The can says it is
water-soluble, but it isn't. This is the wrong product to use for this purpose.
Upside-down Marking Chalk. Untested. This company, found on the web, sells spray chalk in cans that cover 500 ft of linear stripe per can. The company is Time Striping, Inc. It's $40.00 per case (and by the case only). They say it lasts up to 30 days. "Up to" is good news, but after my experience with the last two similar products, I'm not inclined to get a case of it to test it.
Home Made. Maybe someone knows of a better mix than my experiments have yielded so far. When it was one third powdered chalk and two thirds water it wrecked the pump sprayer and left a sloppy stenciled image. A foam roller worked worse. So I made a mix with corn starch as a binder/lubricant. One tablespoon of cornstarch heated in 2 cups of water till it thickened plus two tablespoons of powdered blue chalk (from builder's supply) sprayed better and left an image that was less bad, but still messy. It didn't work nearly as well as Air Chalk. Although, in a pinch it might be better than nothing.
If you have any information about or experience with aerosols or mixes, let me know.
When two colors overlap, the double coat of the overlap
can be difficult for rain to remove
One citizen, tired of looking at someone's message on a sidewalk, tried washing the spray chalk off and could only partially remove it. That became an issue in the news. That would be bad public relations for this cause. Please use the least durable products possible.
Spraying chalk on lawns might eliminate the problem of enduring longer than is desired. It will be gone when the lawn is mowed. (Won't it? It appears to have disappeared from my lawn with one mowing, but dew and rain preceded the mowing. I need to test it in dryer weather).
Rainbow Racing Systems, Spokane Washington, (800) 962-1011 or (509) 326-5470, says that their spray chalk is "Not recommended on concrete, brick or porous masonry surfaces" because it can stain permanently. It is better on asphalt and grass. It appears other brands have the same problem, but don't warn us.
Below is a test of repeated use of the stencil with Air Chalk
(on my driveway)
Right now I am making these stencils for a specific event, the Walk for Patient Safety being run by www.LeapforPatientSafety.org in New York. They are thinking of marking the sidewalk with this symbol along their entire route. Dr. Colantonio is the one who thought of using spray chalk, a great idea, so I am making stencils designed for that and working to learn more about which brands of spray chalk mercifully wash off in the rain.
For them I also made a couple of stencils that can be used for making the sign shown at this link.
By the way, the Walk is billed as being about "errors." That leaves out many victims of "adverse events" in medicine. "Adverse event" is a more inclusive term because not all adverse events are errors, but all errors are adverse events. For instance, if no one will diagnose injuries because they are iatrogenic, the lack of treatment can result in worse injuries that are not errors. Many more would feel included if "adverse events" were the subject rather than "errors."
I will make stencils for you for free if you have an interest in putting patient safety symbols someplace public.
Back to A Symbol for Patient Safety
Please don't use spray paint or anything permanent on a sidewalk or other public place. If it is permanent, it is graffiti and is more likely to alienate than to persuade (and might be illegal). Christo left his mammoth exhibit of 7,500 Gates in Central Park in Manhattan for less than three weeks because it is special and poignant if it is rare and of short duration. Chalk outlines still on the ground after 3 months may work against us. It is important not to be obnoxious to our own constituency.