Avoid vertical surfaces with spray chalk. Rain usually cannot remove chalk from them.
If it is not temporary, it might do more harm than good for your cause.
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 was bad public relations for that cause. Spray chalk is chalk in a spray can. It is supposed to wash off in the rain, but some brands don't. Below are tests I've done on a few brands.
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.
Spraying chalk on lawns would eliminate the problem of enduring longer than is desired. It will be gone when the lawn is mowed.
When I looked (things keep changing) I could not find spray chalk at Wal-Mart or Michaels or Sams. And the building supply companies like Lowes and Home Depot had only pavement marking sprays that were more permanent than I was seeking. However, I did find something at Hobby Lobby.
Air Chalk (seen above). Recommended. I tested a brand of spray chalk called "Air Chalk" distributed by Goodmark. It sprayed well and could be removed with a hose. The tests that I did not remove with a hose disappeared after about a dozen rainstorms. A 3 ounce can cost $1.99 at Hobby Lobby. That is small, but it is pocket-size, which is handy.
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.
Testors now has something they claim dissolves when rain hits it (see it at this link). It sounds good, but I haven't tested it. These products change nearly every year and I haven't kept up with it.
When two colors overlap, the double coat of the overlap can be difficult for rain to remove
Inverted Marking Chalk. Not recommended. The cans are held upside down, as are most pavement marking products except Air Chalk. 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 tried to remove it with a long, hard spray from a hose. The image became fainter but did not wash away. That's not as temporary as I wanted. 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. Also, the strong blast has a tendency to lift the stencil and create rough edges on letters where crisp edges would have been better. Air Chalk is a better solution if you can find it.
Rust-Oleum Spray Chalk. Not recommended except for long term installations. It has overspray problems too, 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 temporary marking.
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 available only by the case). 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.
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 are the same, but don't warn us.