So a ten grand investment on wfp should always be cheaper than your 70p bucket
You never get a great clean first and more often than not second time due to soap build-up on glass from years of traditional cleaning so thinking that you will clean window using traditional method first time is a contradiction - you will only perpetuate the problem. Explain situation to customers without sounding too gimicky - be as honest and straightforward as possible. Bear in mind some customers will accept good naturedly others will react with scepticism. Accept that you will always have a mixture of customers regards those who have no problem with you using wfp and the others who would prefer you to use traditional - but price accordingly. Traditional should always be at least a couple of quid dearer (use of ladders etc...).