The type of bonding mortar used to apply KERDI must be suitable for the substrate and must penetrate and engage the membrane fleece. For all the substrates listed in this Handbook (e.g., gypsum board, cement backerboard, etc.), Schluter SET™, ALL-SET™, FAST-SET™ or an unmodified thin-set mortar is recommended. When installing KERDI with unmodified thin-set mortar over particularly dry and porous substrates, it is recommended to wet the substrate first, in order to help prevent premature drying or skinning of the thin-set mortar. Schluter SET™, ALL-SET™, FAST-SET™ or an unmodified thin-set mortar must be used to construct all KERDI seams to ensure watertight performance of the system.