Slate water line tiles

[KurtV]

Do you have a saltwater chlorinator by any chance?

[captainx]

Yes sir, we do.

(Why am I suddenly afraid?)

[KurtV]

There's a school of thought that says that the saltwater can corrode stone though this has, as far as I've heard, been restricted to coping and decking and not waterline stone.

The theory is that when the saltwater from splashout dries on the stone it leaves the salt behind, which, if it has penetrated the stone through pores or cracks, can cause spalling, cracking, flaking, etc. as it recrystallizes.
I've not heard any accounts of this happening with waterline stone but that may just be because it's not very common to use stone on the waterline.

The solutions most often mentioned are sealing the stone and/or rinsing it off regularly with fresh water. Neither seems a very practical solution on the waterline. I guess you could drain the water down below it and seal it but it sound like you tried that without success.

Slate, from my very limited kowledge of it, is a soft stone that tends to flake. It may just not be a very good material for this application.

There are a couple of threads over at troublefreepool.com on this issue. Try searching on SWG and corrosion over there.