Originally Posted by Skid Row Joe

I would use Compotite Gold anti-fracture, roll-out membrane. Price is less than all the rest, and it's the most tested anti-fracture membrane in the history of the Ceramic Tile Institute - Los Angeles, CA. Google Compotite for a dealer near you.

As far as 'sealing before grouting?' Sure - use which ever sealer you are going to eventually seal the slate and grout joints with. Aqua Mix would be a good choice - you'll just have to determine which one to use - they have several.

Both product lines are highly available in CA. - since they are California-based companies.