I saw your other post and commented there about the tank. It has ben up and running about a year now with no issues at all. Not a single leak and I got it sealed correctly. As far as the slate on the back wall I "think" you will be OK but if I were you I would do some testing. I would get all of the layers of LR you need to seal the tank on the back wall and let that dry. Once dried, I would apply a good amount of LR to that back wall and to the back of the slate and stick them to the wall. You will been the have the back wall laying flat but I figured you assumed that already. I also imagine drying time on the slate will be slow.