How to Replace Glass Aluminum Muntins in a Window

Measure the location of the missing muntin. Compare that measurement against the other muntins on the window and find one the same size. Lightly sand the surface of that muntin with fine-grit sandpaper, which removes debris from the window.

Cover the windowpanes with masking tape. Tear off a small piece of the masking tape and rub it across the window. The tape protects the panes, but will not leave any residue on the windows. Rub a thick layer of petroleum jelly onto the muntin, completely coating the aluminum.

Rub patching plaster onto the muntin. Once the plaster dries, peel it away from the window. This small piece serves as a mold for your new muntin. Set the plaster on a flat surface and run petroleum jelly on the inside, which keeps anything from sticking to the mold.

Mix the automobile filler according to the package directions. Most fillers consist of two parts, which you mix in equal amounts. The mixture should be slightly thick and similar to molding clay. Press the filler into the mold with your hands.

Press the mold against the window on your missing muntin. Line the mold up with the missing muntin and press down. Apply enough pressure that the filler sticks to the window. Let the muntin dry on the window for several hours.

Run the edge of a utility knife around the edges of the mold. Carefully peel away the mold and the filler should stick to the window. Lightly sand the top of the new muntin until it feels smooth and matches the surrounding muntins. Remove the masking tape from the window.