I use scotchgard, but the recommended drying times of 6 - 12 hours puts a lot of people off at the time of cleaning, they then want to re-arrange a future date to have protection when the property will be empty for a long period.
The instructions state that it can be applied to damp or dry carpets, do you think it bonds less effectively to a dry carpet?
Paul
P.S.
I cleaned a carpert yesterday that had never been cleaned, though she did have it scotchgarded when first put down, it had been down for 8 years and it was surprisingly clean. I noticed when cleaning the carpet where the sofas had been the water was still beading up and sitting on the surface so like Shaun says have belief in the product yourself, get a couple of off cuts of carpet and treat one, that way you can give your customers a visual demonstration.