Schluter® PRIMER-U Universal PrimerPRIMER U is Schluter's universal primer a ready to use, water based formula that improves the bond of thin set mortar to substrates that resist adhesion. Where PRIMER PS is specifically for peel and stick membranes, PRIMER U works with standard mortar set Schluter products (DITRA, KERDI, KERDI BOARD) and general tile installation. Use it when your substrate is too smooth, too dense, or has residual contamination that would compromise mortar adhesion.