What should I use to seal/coat interior of stone foundation wall?
I'd recommend using pure sodium silicate or potassium silicate for this. These chemicals will form a waterproof seal that is breathable, preventing moisture from becoming trapped and deteriorating the mortar over time. In addition, they will chemically bond to the mortar and actually strengthen it by introducing soluble silica, sealing the micro-pores in the mortar and reducing alkali-silica reactions and efflorescence. Highly recommended. Here's an example product that you can get on Amazon (no affiliation): http://smile.amazon.com/Rutland-146-Cement-Sealer-Fireplace/dp/B004YEDQOK