I've found the best way is when first quoting the job, I explain how wfp and the water works (leave the water on, carries on cleaning and then dries completely clear) and then say this is an ideal system for this country as it won't be affected by the rain. I say that if I was to use the soap and water method then little bits of soap residue that are left behind could run and leave smears. I only state this point as an advantage to wfp if I have already told them that is the method I will use as I use both trad and wfp.
Also when it has been raining when I've been cleaning or has alot in the last few days and a custie says something along the lines of 'have you managed to get much done or bet you've not got much done) I say something along the lines of, ahh I've been okay luckily as all the jobs I've had have been with the wfp so won't be affected. Or say not too bad, had to rearrange a few jobs I do off ladders but the majority are done with the water system now so can't affect the clean. This is just an added reitteration to customers that I still clean in the rain and it wont affect the clean.
I've only been going a year but this explaination has worked for me even when it has bucketed down. Although I do try to avoid if it is very heavy, but if I start in light rain and it gets worse I just keep going and get it done.
Tom