Is Primer Required Before Waterproofing?

“Can we apply the mastic directly without primer? There will be waterproofing on top anyway.” Builders and private customers ask this question quite often, usually in an attempt to save time and money. The short answer is: primer is almost always essential. Let’s look at why skipping this supposedly “optional” layer is one of the most common causes of waterproofing delamination and leaks.

What Is Primer and Why Is It Needed?

Primer is a liquid compound applied to the substrate before installing roll-on waterproofing or applying mastic. It is most commonly a bitumen solution in an organic solvent or a water-based bitumen emulsion.

One important clarification: primer is not waterproofing. It does not protect against water on its own. Its purpose is to prepare the surface so that the main waterproofing layer adheres securely and remains effective for decades.

What Does Primer Do? 5 Key Functions

• Removes dust from the surface — binds fine dust particles inside concrete pores that would otherwise prevent the material from adhering properly.

• Penetrates pores and strengthens the surface — reinforces the weakest and most friable upper layer of the substrate.

• Creates adhesion — acts as a “bridge” between the substrate and waterproofing, ensuring strong bonding.

• Fills microcracks and surface irregularities — evens out the surface’s absorption capacity.

• Reduces consumption of the main waterproofing material — a primed substrate does not absorb excessive amounts of mastic.

What Happens If You Skip the Primer?

Saving money on primer almost always results in rework. Without primer:

• The waterproofing does not bond properly to the dusty substrate and begins to peel away.

• Air pockets remain underneath the coating.

• When heated by the sun, trapped air and moisture expand, causing bubbles and blisters.

• The membrane or roll material separates from the substrate, allowing water to penetrate underneath and migrate along the coating.

• After 1–2 seasons, the waterproofing may have to be opened up and completely redone.

Types of Primers

Solvent-Based Bitumen Primer (Ready-to-Use)

The most common option. It penetrates deeply into the substrate, dries quickly and provides reliable adhesion. The disadvantages are a strong odor and flammability: it is not suitable for enclosed spaces without good ventilation or for use near open flames.

Bitumen Emulsion (Water-Based)

Bitumen dispersed in water. It has no strong odor, is safer against fire and more environmentally friendly. Suitable for use in enclosed spaces such as basements and bathrooms. It takes somewhat longer to dry than solvent-based alternatives.

Polymer Primer

Used under polymer membranes and mastics where chemical compatibility with the polymer waterproofing compound is required. It is often used on complex or smooth substrates.

Concentrate or Ready-to-Use

Some primers are sold as concentrates and diluted with a solvent on site according to the instructions, while ready-to-use formulations can be applied directly from the container. Ready-to-use primers are more convenient and eliminate the risk of incorrect mixing ratios, while concentrates are more economical for large-volume projects.

Consumption and Application

• Consumption: approximately 0.25–0.45 kg/m², depending on the porosity of the substrate (porous concrete absorbs more).

• Substrate: dry (moisture content no more than 4%), clean and dust-free, with no oil stains or loose areas.

• Application: apply evenly in one coat using a brush, roller or sprayer, without puddles or untreated areas.

• Drying: usually 1–4 hours to become “touch dry” — the surface should not stain or feel sticky when touched.

• Important: do not apply waterproofing over an uncured primer — the result can be the same as if no primer had been used at all.

When Can Primer Technically Be Omitted?

Almost never when working with mineral substrates such as concrete, cement screed or brick. A rare exception is when the manufacturer of a specific self-adhesive material explicitly allows installation on a particular prepared substrate without primer. Even in this case, the surface must still be thoroughly cleaned and dust-free.

A simple practical rule: when in doubt — prime. The cost of primer is negligible compared with the cost of repairing failed waterproofing.

Which Primer Is Needed?

• Concrete, reinforced concrete slab — Solvent-based bitumen primer

• Cement screed — Solvent-based bitumen primer

• Brickwork — Bitumen primer / bitumen emulsion

• Enclosed spaces (basement, bathroom) — Bitumen emulsion (odor-free)

• Substrate for a polymer membrane — Polymer primer

• Metal — Special primer for metal

A Consultation with a MARJA Specialist — Free of Charge

If you are not sure which material suits your project, give us a call. MARJA specialists will select the optimal solution, taking into account the type of structure, the climatic conditions of your region, and your budget.

Phone: +998 77 057 57 53

E-mail: info@marja.uz

MARJA — a manufacturer of waterproofing materials in Uzbekistan since 2004.
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