There are cement based coatings available to seal basements that should work for waht you are trying to do. I don't have a brand name since I last used it 20 years ago. It is basically painted on using a very coarse brush. It comes in various colours, and requires a couple of coats, but it worked very well when I used it. It should work even better, in your case as you are trying to keep water in, whereas, I was trying to keep it out. Check the building stores. The stuff I used was a cement powder, that you mixed with water. Basically, it was similar to parging.