PRIMER-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. It creates a bonding bridge between the substrate and the thin-set.

When to use PRIMER-U

• Dense concrete: Machine-troweled or power-floated slabs that are too smooth for mortar to grip
• Existing tile or stone: When tiling over existing surfaces without removing them
• Non-porous surfaces: Metal, glass, or coated substrates
• Contaminated substrates: Surfaces with residual adhesive, paint, or curing compounds that can't be fully removed
• When unsure: Primer is cheap insurance — if the substrate feels slick or sealed, prime it

Application

• Stir well — do NOT dilute
• Apply one thin, even coat with a roller, brush, or sprayer
• Let dry to a clear, tacky film (typically 30–60 min depending on conditions)
• Apply thin-set mortar while primer is still tacky (within the open time window)
• Room temp 40–85°F, humidity below 80%
• Coverage: ~200–350 sq ft per gallon

Specs

• Type: Ready-to-use, water-based universal primer
• Compatible with: Standard mortar-set DITRA, KERDI, KERDI-BOARD, general tile
• Not for: Peel-and-stick membranes (use PRIMER-PS instead)
• Coverage: ~200–350 sq ft/gallon
• Shelf life: 12 months from manufacture

PRIMER-PS vs. PRIMER-U — which one?

• PRIMER-PS: For peel-and-stick (-PS) membranes only
• PRIMER-U: For everything else — mortar-set membranes, tile, KERDI-BOARD
• They're not interchangeable. Using the wrong one won't necessarily fail, but it won't perform as tested and may void the Schluter warranty.

Pairs with

• DITRA, KERDI, KERDI-BOARD — mortar-set membranes
• Schluter SET, ALL-SET, or FAST-SET mortar