Can a stone sealer be applied on top of this? I plan to use this to repair grout joints in slate tile floor, then apply a sealer over entire floor.

Thank you for the question, Andrew. Sealers can be applied over the Loctite PL Concrete Non-Sag Polyurethane Sealant, but the sealant must be fully cured otherwise you will encounter compatibility issues such as stickiness or discoloration. Based on the description of your project using this product for grouting purposes on a slate floor may not be a good idea and would not be recommended. You may want to do a little further research because there are grouting materials more suitable for this type of repair. We recommend you consult the slate floor manufacturer or supplier for proper materials to be used.