PERSONALLY....I think a Topical sealer for Slate is correct and the right seal for the job. Others may think differently.
Lithofin do Slate Seal, not my favourite seal, but the customer likes the end result. I use Lantania products, In my opinion, there are the best available at the moment, I am sure others have their favourites, you will find Topical sealers difficult to remove when you come to clean it off....but thats a whole different topic, which has been discussed many MANY times before!