Bill, First clear the area of all rubbish,Hire an air compresser and a airhammer (wind hammer) with a flat chisel end with teeth on it, take the surface off the wall by a 1/4", Have two plasterers render the wall with cement made up of sharpe sand and cement 1 of cement and 3 of sand. not too thick, let it dry ( about a week) then render the wall again for the finishing coat. If you only put one coat on the wall, after a time you will see where the joints in the concrete are, and you dont want that ( it looks cheap). Try that and see what happens. John 2000.