I've used cement based fillers like External Polyfilla and Murex but the small boxes/bags are expensive for large areas so I was interested to see external cills etc on a house near here being done using 25kg backs of a cement based filler called CEKOL C-35. Trouble is I can't find it locally and only at one place online
PS FTAOD I am far too mean to use car body filler or the equivalent given the quantities I need to resurface cills.