2. Mask off the glass with painter's tape beside the area you're repairing. Wearing gloves, mix a small batch of epoxy until it's the consistency of peanut butter. Dab a thick coat onto the damaged area with a plastic putty knife.
Now place the spreader vertically on the glass and drag its profiled cutout slowly across the muntin, scraping away the excess epoxy. One pass should be enough for shallow repairs. If the damage is deep, add more epoxy and make more passes. After the epoxy hardens, sand, prime, and paint it.