I agree with you, I would not normally offer this as a stand alone cleaning system, but they did'nt want the additional cost of cleaning this wall, it was over 200m long winding around thier garden and not all in the condition of this test sample.
The choice they had was leave it as it was or use the quat to give it a visual clean.
But which would stay the cleanest longest? Pressure wash it with no residual protection or use the quat which should keep working over the coming months.