Hello Emerald 76,
I built a Slate cave which turned out i think really nice! i did make sure the slate was well prepared like allowing it to soak for at least 7 days then gave it a good "cooking" which the neighbor's thought very strange! i then Built the cave, i used Black Gold Label Aquarium Sealer which in my opinion would hold a house together.
If that Sealer isn't available then I'd use which ever well known aquarium Sealer you have in your area. after the cave was built i then gave it anther good soaking for i think a week.
its been in my tank now for some time the fish just love it, theres a good growth of moss now taking shape on it i couldn't be more pleased with it ...... so i say "Go for it" your fish and you can then enjoy your skills.