In concrete or stone, cracks wider than 3mm or those that appear to go all the way through may indicate structural movement and might require professional injection or replacement. How to Repair a Wood Window Sill
: If rot is present, use a chisel or oscillating multi-tool to excavate back to bright, healthy wood fibers. Stabilize & Fill : repair cracked window sill
The success of the repair relies entirely on the preparation of the surface. In concrete or stone, cracks wider than 3mm