Superhydrophobic coatings, like NeverWet, repel water to a very high degree. It does this by reducing surface tension by creating a very thin layer of air above the coating. Water beads up to almost a perfect sphere and rolls right off. The area around the image darkens in the rain, revealing your drawing.
You can use this to make innovative street art that only shows up when it rains or gets wet. (Make sure to get permission and follow all applicable laws in your area)
The downside of NeverWet is that it isn't perfectly clear: the surface becomes a translucent, frosty white that shows up on dark surfaces. However, it's nearly invisible on light-colored concrete.
Don't worry, it's not slippery at all. The surface of the coating feels indistinguishable from the texture of the concrete.
Here's what you'll need:
NeverWet - read the label for coverage to know how much you'll need. I did both stencils you see in this Instructable with one set of NeverWet cans. You can also source some industrial suppliers, which is what I was using before NeverWet became commercially available.
Drawing - Use your imagination!
Stencil - I used matboard because I had some around the studio, but it's not very durable. You can use chipboard cardboard, corrugated cardboard if you don't need a lot of detail, thin plywood or whatever else you can make a stencil from.
X-Acto knife - Or other means of cutting out your stencil, like a laser cutter or even a scrollsaw for plywood.
Spray Tack - (optional) I used this to get the stencil to stick to the ground so the edges wouldn't curl up. I got much cleaner lines this way.
Check out the videos for visual step-by-steps.
Step 1: Stencil
The first step is to make your drawing into a stencil.
I did my text and drawings in Illustrator, then printed them out at 1:1 scale using Illustrator's Tile setting (search Illustrator's help section if you're unfamiliar with this feature). I taped all the sheets together and then glued the paper to the matboard with spray glue. This gave me my template which I then cut out by hand. If you have access to a laser cutter this step will be much less aggravating.
I made one stencil as a positive ( I'm Only Happy When It Rains) and one as a negative (The Sun'll Come Out Tomorrow).
To arrange the pieces for the positive stencil, use the negative sheet you just cut from as a placement and registration guide, then spray tack the back of your pieces and stick 'em to the sidewalk (I did mine by laying them all out face down on some scrap newsprint and spraying the backs of everything at once).
I also used some scrap plywood as a border to contain the overspray.
Step 2: Spray
NeverWet is a 2-step process. A base coat and a top coat. Follow the instructions on the can label for coverage application and dry times between steps. Spray the first step evenly, wait for it to dry, then spray the second. You may need additional coats; again, follow the can's directions.
As you can see in the second image, the coating is faint, but basically invisible if you didn't know to look for it.
Step 3: Make It Rain!
Wait for it to rain, or splash some water on your new Rain Drawing!
The coating is rated to last for a year, then you may need to reapply the top coat. I made these drawings about eight months prior to writing this, and they're still there. They've gotten dirty and are not quite so invisible anymore, but the coating still works when it rains.