My mum has an Airedale like me, called BB, and she has suffered from staining - she is a tan coloured dog but it still seems to stain just below where the pee would flow.

Her vet advised that some dry dog food can cause this as it makes it more acidic or something; however the vet still reccomended a dry food which did not change things.

My sister is a dog groomer and uses a spray called "the stuff" which you spray on the coat while wet. It seems to coat the hair and make water and wetness bead off rather than stay on the fur. It keeps the coat really nice. On white dogs she will spray all over the coat but a bit extra around the bottom area and wipes some just under the eye for tear staining prevention.

She lives in America so I am not sure if you can get it over here but im sure groomers know of something similar here! I just found a website for it in the US: Dog/Pet Grooming Supplies - Grooming Sprays