I have a natural stone foundation and want to finish my basement eventually. There is currently a humidifier in the basement left from the previous owner to control the moister. There are no leaks but the walls sweat and mildew/mold forms when the de-humidifier is not running.
How can I seal the foundation to control that moisture issue? I know I can put a barrier when I put the walls in but I want to fix the moisture problem before I do that if possible.
You won't seal this foundation no matter what you try. What you need to do is control the moisture and condensation. Does it condense to the point it runs down the wall onto the floor? Is the floor level or does it slope to one side of the house? Where do you live and how humid does it get in the Summer? Will the finished basement be heated/cooled?
Let's start there.
From what you said, the stud wall will be in from the foundation wall enough for air flow behind it. You could need an exhaust fan to move the moist air that's behind the wall, out of the house. Maybe vents in the sheetrock wall to allow the moist air into the room so it can be dehumidified. Maybe a fan or two pushing the moist air behind the wall. Options abound.