Can anyone give the definitive cause of this problem. I'll take the job i did yesterday as an example. They are dark marks. Many as spots, some as larger marks about the same area as a saucer. When you attack them they turn from a dark colour to a soapy white colour and are more or less removed. They can be all over a house and werent caused by the occupier of the property. But also, they tend to return later, maybe not as visible but they can return. Coincidentally, I've come across these when a property has changed hands. I'm guessing maybe something like Vanish has been used by the outgoing tenant to remove marks when trying to sell property. Any ideas out there?
Chris