The house is 90 years old with 300 m.m solid walls rendered.All windows have conctete cills 800m.m wide by 100 m.m deep.Some of the cills have vertical cracks right through the cill.Have tried in the past veeing out cracks and filling with various types of morter and plastic fillers but eventually the cracks return.Is there a way of fixing these cracks once and for all?There are no structural problems just want to repair for cosmetic reasons