I cheated, didn't bother patching - put on a good layer of seal (which is just like shellac varnish) to bind the dusty rendering, then sprayed on a few coats of Sandtex Masonry paint, really impressed with that paint - very dear but sometimes amazing deals available. If rendering is relatively smooth you can roller or brush it on (with special masonry brushes/rollers).
I found putting two thick coats on was better than four thinnish coats.
In a time of universal deceit - telling the truth is a revolutionary act. George Orwell
When the debate is lost, slander becomes the tool of the loser. Socrates