When my table was installed they did not use anything on the joints. I asked why not and he had me close my eyes and slide my hand across the joint. I really had to focus to feel anything. He did use very thin wedges under the table to make sure the slate came together perfectly. Once the felt was installed, I could not fine the joints. The imperfections in the felt was much greater than in the slate.
If you don't want the joints you could always get a one piece slate and try and carry it in and put on your table. For an 8" table that would be approx. 900 lbs.
