How to Repair a Reentrant Crack on a Window

Insert the tip of your chisel into one end of the crack. Use a hammer to lightly tap one end of the chisel to chip any the jagged edges and widen the crack until it forms a V shape. Repeat this technique along the length of the crack until you reach the opposite end.

Attach a hose to a shop vac. Switch on the vac and vacuum out any dust or debris in the crack. Prepare quick-setting joint compound in a bucket, according to the manufacturer’s instructions.

Scoop out the compound with a trowel and apply a thin layer in the crack. Cut a strip of drywall paper tape to size with a scissors and position it over the crack while the compound is still wet.

Combine patching compound with water in a bucket, according to the manufacturer’s instructions. Use a clean trowel to scoop out the compound and apply it over the tape in a thin layer. Allow the patching compound to dry, according to the manufacturer’s recommended time frame. Use a dry/wet sanding sponge to sand over the dried patch until it is flat and even with the surrounding wall.