Kevin,
Put some enrich and seal on a stone sill in a bathroom which has gone uniformly very dark (not patchy at all). I used the product in other selected places in the house with no problem. The house had been derelict and is still being renovated. The sill was a random piece of sandstone from the original building. It is a 'through' stone so the outside edge is on the external wall and the rest is visible inside the bathroom basically the window frame is sitting on it. I think some damp is being pulled through the stone as there was a small dark crescent shape visible just below the window frame.
This is not an end of the world problem as I have just been given the job of sorting out the porcelain tiles in both bathrooms after a contractor has dropped something all over both. Just wondered if there was anything you could do to lighten the stone up? I thought may be sealing and coating remover with a good dwell time and possibly repeating the process. What do you think? I assume that its just absorbed lots of the product as its a particularly porous piece of sandstone.