cracks in external window sills

[Burnz0]

Hi All. The exterior concrete window sills on my house have seen better days and so today i decided to start working on them. Its a bay window and on the corners there are several cracks some of them a good 10mm or so wide. The paint was badly flaking and in some areas it looks like someone has used some kind of polyfilla to fill and even out some rough spots. Ive been stripping the paint off with a scraper and the majority of it is now back to the concrete but the areas just under the upvc window sill are proving much more difficult and its too close to the window to get a heat gun in there. Is it okay to just rub these bits down so there are no rough edges and leave them as they are or does it all need to come off?
Secondly, for the cracks am i best using a polyfilla type substance again or a morter mix? My plan was to dig out the cracks a bit more first and then fill from them regardless of what i end up using?
When they are ready for painting do i need to put anything down first or just paint straight onto the bare concrete and will one or two good coats do?

 

[shef61]

You want to use proper concrete repair which you can buy from DIY shops, you buy a 5kg or so tub and mix it with water. To fix the cracks chisel out any loose material and fill it as per instructions. To paint concrete you want to prime it and use a suitable paint. Some exterior paints specify concrete, masonry paint or exterior paint would be the second best option but some with react with the concrete and flake off in time. Make sure all the old paint is off before you redo it, the old paint will make the new just fall off. When I did mine i used an abrasive pad on the paint when the scrapper wasnt working.