if there is large damaged areas you need to fill these with sand and cement but first apply a coat of pva glue like uni bond straight from the tin to the area fill with soft sand and cement a 5 parts sand 1 part cement once this has set you can cover the whole cill, paint on pva mixed with water 3 to 1 let dry for an hour and rub on your cement mix with a piece of wood in a circular motion once dry it can be painted
john cavill
October 2008