most on here know my views on working in the rain, the customers ( who pay you ) DO NOT LIKE IT, they might accpet it, but they are not over the moon about it
Matt with respect you are clearly unwittingly letting your customers dictate how you work. See my responses above about how I overcome this objection. You need to bite the bullet & start training them. I have never lost a customer due to working in the rain. If you wre to walk away and it rained 1/2 hour after you have been, whats the difference, sorry my friend but I think you need to man up over this 
to be fair i do let customers dictate how i clean windows, its not unwittingly either, as they pay me the money, so if they want the lounge window sqeegee'd off after i WFP it, then they get it done ( they pay for it though, i now have 3 who want this )
now i know people scoff at this, but they are the customer, i provide a service THEY WANT, if i decide to provide a service i want to provide ( and ignore the customers requests ) then they will go elsewhere, yes most will stay, but my round has been built so that i do house after house after house in the road ( most are 80 % plus ) this suits me, i accommodate reasonable requests
we run a biz that provides a service, i try 100 % to provide a service that the customer wants / needs, if they want me to whistle george formby whilst i clean the french doors at the rear, then whistle i will

i run a pretty low risk strategy like this, it works for me