I don't know if there is a special paint to use, but can tell you my experience with 'screens'. In order to tune and properly focus some diode lasers, I needed a 'target' which would accurately reflect the beam back from about 40' away. Trying various white papers and mirrors proved useless. The solution was literally lying at my feet. While driving one day, I noticed that a road crew had recently painted some yellow and white lines on the road and had coated the stripes with tiny glass beads, similar to those used on older movie projection screens. I scooped a couple pounds of the leftovers and rushed home. To make a long story short, I painted a 4' x 6' panel with FLAT white enamel (gloss doesn't work as well because it disperses the beam in a random manner) and sprinkled a heavy coating of the glass beads on it while the paint was still wet, then rolled it with a smooth rubber roller to firmly embed the beads. Once dry, the panel was gently vacuumed to remove any excess glass and placed against the far wall. Viola! Success! The reflected beam was clear, bright, collimated, and totally acceptable for my purposes. I can't see why it wouldn't be equally good for a projected picture. How do you get the beads? Contact your local highway/streets department; they can tell you who their supplier is or maybe tell you where their next project is so you can go and pick up their 'scraps'. Now the question is: How do you get it onto an already installed vertical wall? You could try a sandblasting gun attached to a compressor operating at very low pressure, or you could just try throwing handfuls of the stuff against the painted wall, OR.... OR.... (Are you ready for this??) OR you could tip your house over 90 degrees so the wall is now the floor and proceed as I did with the panel. ;D
