Krend- patching under window

[rhysevans27]

Hi there I know patching any mono render can sometimes almost be impossible but I have been asked by a window fitter to try and repair under the window. It will be hacked off and a clean cut from the window sill down around 400mm to the bead. I have made him aware that mono upto mono with out a bead is risky and might still see the join but he has no choice other than to hack the whole front off the house off. Just asking to see if anyone has been successfull in doing something similar? I thought about duck taping up the existing so I don't fill it but welcome to anymore ideas.
Cheers

 

[Rigsby]

Make sure the edge is full depth and square. Treat the thickness edge with a primer of sbr to avoid suction. Use masking tape accurately up to the edge then some wider tape on that. Fill in and leave proud. Bar back to 1mm proud then just before your last scratch back remove the tape being careful not to scratch the surface of the existing. Dust down.

 

[rhysevans27]

That's pretty much how I thought about doing it to be honest. Cheers for your help

 

[Topspread16]

Cut with a multi tool, not a grinder. Grinder with leave dust on the surface also shards of steel which can later rust stain.