lifting render

[ken the chef]

The render to one of my outside walls [gets all the rain] has lifter around the window. Tap, and it sounds hollow plus a few small cracks.
The Bungalow was built about 1986. As yet I do not know what type of block or brick are behind the old render.
It's obviusly requires reairing but I am not sure what rendering mix to use. I have found several webb sites that give conflicting mixes ranging from 1 to 3 / 1 to 6 / plus some with lime.
I intern doing the job myself.
Any info will be apreciated

 

[jbonding]

your scratch coat should be 6-1 and your top coat should be 4-1-1, put your first coat on flattening out the wall then scratch it horizontally, then your top coat flattening it using a straight edge fill any slacks and rub it up with a plastic float then lightly rub over with a sponge. If the surface is dry and dusty wet it or mix pva and cement wet and brush onto the surface.