I have a similar customer, been doing them for 6 years. Wife lovley lady, shes the one who wanted a window cleaner, husband to start with wouldn't even talk to me or acknowledge me.
One day I overheard them arguing about using me, he was objecting to her having to pay someone to clean the windows when he can do it himself. Her reply to him was, well if you actually DID clean them like you said you would I wouldn't be paying for the service of a window cleaner to start with; but in the 20 years I have lived here you have never actually done them amongst other things I wont mention..so we are keeping him. lol
As time went on though he softened the day he grunted to me in acknowledgement I nearly fell over took about 4 years though.
Then one day I knocked on the door, and wife wasn't in, hello sir I have finished cleaning your windows here is your bill. He sort of grunted to me 'I suppose ill get the money hang on' he comes back hands me the money and says to me how comes when you use your bush thingy the water dries off and leaves no marks on the glass?
I explained to him that we use de-ionised water, showed him the setup he asked me how do I know my water is pure? my reply was well I test it using a tds meter, if it reads 0 its pure if it goes to 1 or above it the resin needs to be changed. How can you trust that tds thingy he says. I say because it takes two measurements, first it measures the voltage across the water, it then compairs the resistance with the terminal voltage and you get a figure in microsemens which is a measurement of conductance; 100% Pure water is in fact an insulator to electric charge, so the less conductive the water the purer it is then the final figure in microsemens is converted to TDS total dissolved solids which is an calucalted estimation of howmany dissolved solids per million are in the water. I want 0 so I keep it at 0 and change it as soon as it goes to 1.
He was very impressed with that, and it turns out he used to work for Marconi as a radar specialist in the 1970's
from then on we get on fine..