Hi,
We use a granular bentonite product that is commonly used in the drilling industry. Benseal is one trade name, 8/25 mesh bentonite is the spec. When mixed with water it creates a watertight seal that has the consistancy of very thick peanut butter. It never dries out and can re-hydrate for its entire life. Cost = 12.00 for 50 pound bag, enough to seal about 50 holes.
Eric