How to Paint Styrofoam Grout

Wait at least 24 hours after applying any glue to the Styrofoam before you begin the grouting process to allow time for the glue to set.

Create a stone and grout surface from the Styrofoam using the tip of a hot foam cutter tool to make the grout lines. Plug the tip in and allow it to heat enough so that when placed against a piece of scrap foam it slides through slowly but steadily. Run the tool over the foam in the positions of your grout to remove the foam and leave grout lines behind. Use a light touch only, as you'll use paint to accentuate the lines. To make the lines straight, consider using a metal or wooden straightedge, being careful to not get melted Styrofoam on the straightedge. Make sure the room is well ventilated when you use the foam cutter to avoid inhaling the fumes from the foam. Wipe the straightedge and the foam cutter clean.

Mix a thin batch of gray unsanded grout in a bucket. Add water to the mix until you can paint it onto the Styrofoam with a paintbrush.

Paint the grout over the entire Styrofoam stone surface. Make sure you get the paint into the modeled grout lines cut into the Styrofoam. If the lines were created by someone else, they may be of varying sizes and texture, depending on the sculpting tools made to turn the flat Styrofoam panel into the semblance of stone and grout walls. So add a bit of extra paint where needed to get into the grout lines, but stop short of saturating the Styrofoam. This first layer will begin to turn the foam gray, hardening the Styrofoam as well on the outside of the foam to prevent breakage. Allow the foam to dry for about an hour, then apply two more layers, allowing an hour's drying time after each coat.

Mix a final batch of the grout with the addition of black acrylic paint to darken the color. Thin the grout even further to the consistency of soup. Brush this colored mix over the Styrofoam as you did the last three layers. After coverage however, blot away as much of the darkened grout as possible from the faces of the Styrofoam stones with paper towels, leaving the mixture in the grout lines only to serve as a visible grout shadow against the blocks. Allow the wall 48 hours' drying time before use.