Rendering underside of lintel

[Notsogranddesigns]

I am in the process of putting in a doorway in a 9" solid wall.

Have got the lintel in place, and rendered the sides of the opening but am really struggling with the underside of the lintel as the render doesnt stick at all.
It either falls straight off or if it does stay on then once dry it falls with just a light tap
Have tried attaching some chicken wire to the lintel but didnt help.
Mixture I was using was concrete/sharp sand and tried with/without lime

Any ideas welcome.

 

[northernwarrior]

Gripfill some plasterboard on it

 

[Magrah]

Sharp sand wont stick to anything, try soft sand with a bit of Unibond mixed in.

 

[Richard C]

Gripfill some plasterboard on it

By far the easiest way; use 2 layers to reduce chance of cold spot.