What are you hoping the final appearance will be?

No matter what you use, the slate must be dry dry dry or it will cause efflorescence.

I would like to protect it, bring out the color and add just a little shine.

You can pick up something at Home Depot, but ask one of the salespeople there to make sure you purchase the right product.

I used Home Depot slate and the guy recommended their premium sealer. It's been great! I have slate in my conservatory with plants on it all winter. We have a high calcium in water so there were spots on the floor when I moved the plants back outside. Came right up because of the sealer, I was sold. Be sure you can use it outside.

Thank you!