Uma.... I would scrape all loose matter off with a putty knife sand with a #80 grit sandpaper and repair with "BONDO" that is the brand name for... light weight plastic body filler... it is used on cars to repair(fill) dents... It is a 2 part process that is controllable by the user... mix to a pinky/grey and you have more open time than mixing it to a salmon colour... mix smaller ammounts and apply to the damaged area filling in all the "bad" parts as you go.. this can be done in steps till it is all repaired...after 24hrs. the surface can be sanded and painted with Latex or oil based paint and it will stand up to just about anything..... I know because I have done this to 2 of the 3 sills on my South side (the weather side) and they look great. ... Good Luck !