we would not recommend using ECO or W on damp stone, it would not be an issue (still not recommended) , but can compromise the longevity and performance of the product.
W68 yes, states can be used on damp stone, on the tech sheet application; states to be applied to a clean dry surface.
We tend to recommend the stain stop ECO for the more porous stones and the stain stop W for the denser harder stones - granites etc.
it is a great product (as is the w68) (easy for me to say ) but i do use generally now on contracts as safer for client, but also i don't feel grotty after applications.