Slate tile outdoors---proper sealer?

[TractorLegend]

I have a never ending outdoor deck project. It is a 400~ sq ft deck that 5/4 ext. decking, cement backer board w/ taped and mudded joints, an applied membrane system over that and guaged slate tiles adheres with thinset and grouted joints.
The normal sources say use a waterbased sealer so the stone "breathes"

I like the look of some of these urethane clear sealers like I saw applied on a stamped/ colored concrete slab project.
Can this be used on stone?

I need maximum waterproofing as the drains I have on my deck were installed then reinstalled due to water getting around the outside of the pipe/drain below the deck, its an outdoor deck- uncovered.
Apparently water is absorbed thru slate and is pretty porous.
May have not been the best choice- but it seemed a good solution to covering up some failed/peeling Spantex coating that was existing (Gaco type stuff on decking).
Input from tile/masons or the like appreciated!!

 

[inveresk]

Slate isn't particularly porous as stone goes which is why it's so effective on roofs. Traditionally, grate builders use 3 in 1 oil for finishing slate hearths. It renders it resistant to marking and staining and helps waterproof it. You could also use a cocktail of boiled linseed oil and paraffin, a traditional finish to seal porous clay floor tiles. Whatever you use, best to try a test piece first then if it works ok, slap it on.