Masonry window sill repair after removing the loose material and got it clean,,, I used TOUPRET touppreith- f in several layers ,,ITS NOT CHEAP ,,BUT IT HOLDS ITS SHAPE AND DRYS REALLY FAST YOU DO NOT NEED A FORM. and I used fiberglass joint tape as reinforcement for the big patch in the front of sill you can do this with regular mortar but you may need some kinda of form to keep it in place that could be cardboard and duct tape