I have also similar experience with japanese hagi yunomi. The piece is truly beautiful with natural cracks all over the glaze on the surface. However it has many leaks due to this effect. You may have a look at its surface texture.

I've tried everything starting from japanese traditional method such fill the piece up with warm but highly viscous rice porridge and seaweed; leaving it a day or two and dried it up for a day or two.
Upon pouring warm tea into it, the piece started to leak again. The whole process was repeated 4x and at the end I gave up. As the piece is very nice one, I keep it only for aesthetic purpose, nothing functional.