One or more pictures of the problem will give you much better and more qualified answers

I have sealed cracks is various materials with Epoxy, depending on your problem it may well work.
I repaired a cracked (in two halves) stone basin (bird basin... i think they are called in English??) by cleaning the inside by sandblasting, and then laminating in a layer of glassfibre with epoxy (West Systems 105 with UV stable hardener)
I "glued together the two halves with the same epoxy ..
When filled with water, you can not tell that the inside is coated with epoxy, but the crack is faintly visible all around.
"Sapere Aude"... Dare to KNOW!
The written word is the only truly efficient vehicle for transmitting a complex concept from mind to mind...
103% of all people do not understand statistics...
Do not adjust our mind, theres a fault in reality :-)