How to Make Styrofoam Window Surrounds

Measure the length and width of the window with a tape measure. Purchase foam blocks long enough to fit the measured lengths. Make certain the foam blocks are wide and deep enough to be cut to the desired surround molding specifications. Purchase several extra blocks to use for experiments before cutting your actual surround.

Purchase a hotwire foam cutter with an arm spread that's wide enough to cover the width of your proposed surround molding. Shape stiff foam cutting wire to the shape of the face of your surround molding by hand. The wire will hold the shape when cutting through the foam, but due to the heat, the foam will draw away slightly from the wire, so make the molding lines thinner than your actual desired widths, to allow the foam to shrink away into the desired shapes.

Screw the wire between the arms of the hotwire foam cutter using a Phillips-head screwdriver. Make certain that the wire is positioned so when it's drawn across the foam it leaves the desired face shape behind.

Plug in the foam cutter and turn it on. Allow it to heat up for about three minutes. Put on heat resistant work gloves and open any windows and doors to allow for the quick disbursement of the unpleasant fumes generated when cutting the foam.

Make test cuts on the experimental foam blocks with the wire by drawing the wire down the face of the foam. Examine the resulting molding lines to determine if you’ve achieved the desired look with the wire. If not, unplug the cutter, allow the wire to cool down, then reshape it as desired. Continue testing until you achieve the look you want.

Cut the actual foam pieces that you wish to use as your surround with the hot wire cutter. Drag the heated wire through the foam block, removing the unwanted foam to leave behind a foam face in your desired shape. Turn off the cutter and wait 10 minutes for the foam to cool.

Sand the surface of the foam with 400-grit sandpaper to smooth any roughness created from the cutting process, then wipe the surface with a tack cloth.

Create a hard coating for the Styrofoam to prevent breakage or denting in the foam surface by mixing 1 cup of plaster joint compound with 1 gallon of latex primer and two tubes of painter’s latex caulk in a bucket, stirring the mixture with a wooden stick for three minutes.

Brush the coating mix over the Styrofoam with a paintbrush, covering it completely by using two thin layers of the mix. Wait two hours for the coating to harden over the foam.

Sand the surface of the coating with the 400-grit sandpaper to remove the brushstrokes left by the coating application. Wipe it clean with the tack cloth.

Paint over the coating with a latex exterior paint using a new paintbrush. Allow the paint to dry for 48 hours.

Brush two or three layers of urethane finish over the water-based paint to keep the paint from running when it's rained on, before mounting the surround molding around your window, using construction cement along the rear of the foam. Allow each layer of finish to dry overnight before applying the next layer.