As a general rule you can repair a stone chip that is under the size of a twoonie. Repairs are most successful if they are performed as soon as possible after the impact. If too much dirt gets impregnated in chip it contaminates the chip and makes repair more difficult. It is generally not advisable to repair a chip directly in sight line especially if it is large. A properly repaired chip will last forever and actually increases the strength of the glass in that area. All Cooper Glass stone chip repairs carry a lifetime warranty* against breakage and will be NEARLY invisible after the repair has been completed.