Advice needed to repair flaking masonry

[Hungrybutterfly]

I'm in Scotland and I have some mullions and sills falling apart and I'm not sure what service I even need. You can see from the second image the entire top of the sill would come away if I knocked it hard enough so there's quite a bit needs to be taken out and rendered. Someone suggested just patching it up with a silicon based solution but I feel there's just too much missing and coming away for that to be viable. I don't need a perfect colour match as I'll be painting it once complete. I couldn't find much online about a DIY solution. I'm a little bit handy so I could possibly attempt a repair myself but I'm not sure what kind of render mixture I'd need.

 

[KenGMac]

Hungrybutterfly, good evening.

Looks as if the cracking is occurring on a retro-applies skim coat of sand / cement.

If you look @ image 2 you will see that the " top surface" of the sill has been allowed to overshoot the vertical part of the sill.

Most common reason for such types of failures is rain water permeating behind the applied render and saturating the underlying [Concrete???] sill and verticals.

Ken/

 

[Hungrybutterfly]

Hungrybutterfly, good evening.

Looks as if the cracking is occurring on a retro-applies skim coat of sand / cement.

If you look @ image 2 you will see that the " top surface" of the sill has been allowed to overshoot the vertical part of the sill.

Most common reason for such types of failures is rain water permeating behind the applied render and saturating the underlying [Concrete???] sill and verticals.

Ken/

Thanks for the response. Can you suggest a course of action? I presume I'll need to take off anything loose, clean up and apply a new render but what would that render be? Another mix of concrete and sand?

 

[Deleted member 292770]

https://www.bostik.com/uk/en/catalog/product/construction/emea/uk/cementone-render-repair-mortar/