Waterproofing Primers Explained: Choosing the Right One for the Job

Understand what each waterproofing primer does and when to use it. This quick guide and cheat sheet make choosing the right primer simple for any job.

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Close-up of a stick roller sleeve applying a milky clear waterproofing primer to a porous concrete surface, showing glossy sheen and fine roller texture.

When it comes to waterproofing, most people focus on the membrane — the “hero” of the system. But ask any professional, and they’ll tell you that a membrane is only as good as the primer beneath it. The primer is the invisible bond that makes everything else work. Without the right one, even the best membrane can fail.

At H2O Supplies, we want to make it easy to understand how primers work, why they differ, and how to choose the right one for each job.

What Exactly Is a Primer?
A primer is a preparatory coating applied to the substrate before waterproofing membranes, adhesives, or coatings.

It performs three crucial roles:

  • Promotes adhesion: It bonds the membrane securely to the substrate.

  • Seals the surface: It reduces porosity, preventing air bubbles and inconsistent absorption.

  • Stabilises the base: It binds loose dust and creates a consistent, clean surface.

Different primers achieve these goals using different resin chemistries — and that’s where things start to vary.

The Two Sides of Primer Classification
Every primer fits into two broad categories:

Table comparing two types of waterproofing primer classifications — solvent system (water-based or solvent-based) and resin family (Acrylic, Epoxy, Polyurethane, SBR).

Most modern waterproofing primers are water-based for easier application, faster drying, and low odour. But the resin family — not the carrier — determines how the primer behaves and which membrane it suits.

The Common Resin Families Explained

Acrylic Primers (Most Common)
These are the workhorses of the waterproofing world. They’re single-component, water-based, and fast-drying — ideal for sealing dry, porous surfaces such as render, screed, or plasterboard.

  • Best for: Dry, absorbent substrates

  • Pairs with: Water-based acrylic membranes

  • Examples: Mapei Primer G, RLA Porous Primer

  • Tip: Never use acrylic primers on damp or moisture-affected substrates — they’re not waterproof barriers.

Water-Based Epoxy Primers
These come as two parts that react chemically to cure. They bond to damp or green concrete and create a moisture-resistant film, making them perfect for below-ground or high-moisture areas.

  • Best for: Damp concrete or high humidity environments

  • Pairs with: PU, epoxy, or acrylic membranes

  • Examples: Gripset E60, Mapei Planiseal MR

  • Tip: Use where rising damp or residual moisture is an issue.

Polyurethane (PU) Primers
PU primers penetrate dense or non-porous concrete, providing strong adhesion and flexibility. They come in both water-based and solvent-based forms.

  • Best for: Dense or smooth surfaces like burnished concrete or metal

  • Pairs with: Solvent-based or hybrid PU membranes

  • Examples: ACTFLEX 700 PU primer, Sika WP PU WB Primer

  • Tip: Great choice under high-performance PU membranes or when durability is critical.

SBR (Styrene-Butadiene Rubber) Primers
SBR primers are essentially latex bonding agents. They improve flexibility and adhesion when mixed with cementitious systems.

  • Best for: Cementitious membranes or screeds

  • Pairs with: Flexible cementitious coatings and mortars

  • Examples: Ardex WPM 265, Gripset 11Y

  • Tip: Commonly used in bathrooms or over screeds before tiling.

Water-Based vs Solvent-Based: What’s the Real Difference?

  • Water-based primers use water as the carrier. They’re safer, cleaner, and ideal for most internal or light external work.

  • Solvent-based primers use organic solvents (like xylene) for deeper penetration and stronger bonding to dense, low-porosity materials.

You’ll usually reach for water-based primers on standard building substrates, but solvent-based versions are still preferred for heavy-duty or industrial environments.

How to Choose the Right Primer
When advising customers or setting up a system, ask three quick questions:

  1. What’s the substrate condition?

    • Dry and porous? → Acrylic primer

    • Damp or green concrete? → Water-based epoxy

    • Smooth or dense surface? → PU primer

  2. What membrane are you using?

    • Water-based membrane → Water-based acrylic primer

    • PU or hybrid membrane → PU or epoxy primer

    • Cementitious membrane → SBR primer

  3. Is there moisture present?

    • Yes? → Epoxy or PU primer

    • No? → Acrylic primer is sufficient

Need a quick reference?
Use this chart to match your primer with the right surface and membrane type:

H2O Supplies waterproofing primer selection cheat sheet showing four primer types — acrylic, water-based epoxy, polyurethane, and SBR — with matching substrates, applications, compatible membranes, and key purposes.

Real Trade Examples

  • Internal wet area (bathroom): Acrylic primer on dry screed before applying a Class III acrylic membrane.

  • Balcony or external slab: Water-based epoxy primer followed by a PU or hybrid membrane for UV and moisture protection.

  • Basement or retaining wall: Epoxy moisture barrier primer, then a solvent-based PU or two-part membrane system.

Key Takeaway
Think of primers as the link between the substrate and the membrane — the handshake that determines how well the system performs. They may look similar on the shelf, but their chemistry defines everything: how they bond, how they dry, and how long your waterproofing system lasts.

In short:
- Water-based describes the carrier.
- The resin family defines the performance.

Choose wisely, and your membranes will do the rest.

Ready to Pick the Right Primer?
Browse our Primer Collection to find what you need for your next project. Or drop in-store to chat with the team about membranes, prep products, and system compatibility.

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