The invention relates to a method of conducting a perf wash cement (“P/W/C”) abandonment job in an offshore oil or gas well annulus (2), in particular the washing or cementing operation using a rotating head (6, 8) with nozzles (7, 9) dispensing wash fluid or cement at pressure. Certain values of parameters of a washing or cementing job have been found surprisingly to affect the quality of the job, or the degree to which they affect the quality of the job has been unexpected. These include including rotation rate of the tool, the direction of translational movement of the tool, and the volume flow rate and pressure per nozzle of cement or wash fluid (and hence nozzle size).