Quartz onto ply

If you’re using UltraTile ProFlex S2, you should follow the manufacturer’s instructions. This adhesive is already a flexible S2-rated adhesive and is normally mixed with clean water, not latex. However, the more important question is whether the plywood substrate is suitable. Plywood must be exterior grade, securely fixed with minimal deflection, and properly prepared before tiling. Many tilers would recommend using a suitable primer on the plywood before applying the adhesive. Adding latex when the adhesive is designed to be mixed with water can affect its performance and may invalidate the manufacturer’s specification. Unless UltraTile specifically states that the product can be mixed with a latex additive, stick with water. Given that you’re laying 600x600 quartz tiles, I’d also make sure the floor is rigid enough, as large-format tiles are less forgiving of movement than smaller tiles.