Plastering and Rendering Primers: SBR Slurry, PVA and Bonding Agents

Before plaster or render goes on, the wall often needs a primer or bonding coat. Get it right and the plaster or render grips well for years. Get it wrong and it can blow, crack or peel. This guide explains, in plain words, what SBR, PVA and bonding primers are, how they are mixed, and where each one belongs.

RCR Connections Ltd is Birmingham based, covering the whole of the Midlands. Our team has 62 years of combined experience with plaster, sand and cement, and K-Rend monocouche and silicone render, and we choose primers on every job.

Why walls need a primer at all

Plaster and render are wet when they go on. How well they grip depends a lot on suction, which is how fast the wall pulls water out of the mix.

• High suction. Thirsty brick, old dry plaster and some blocks suck water out fast. The plaster or render can dry before it has set and bonded, leaving it weak, cracked or powdery.
• Low suction. Smooth concrete, engineering brick, painted walls and old shiny render hardly take any water. Wet plaster or render can slide or fail to grip.

Primers and bonding agents even this out. They slow down a thirsty wall, or give a smooth wall something to grip.

What SBR is

SBR is a milky liquid made from a type of synthetic rubber. It is mixed with cement to make a bonding slurry, or with water to make a primer. Once it has cured, it holds on well and resists water, so it is used outside and in damp places as well as inside.

SBR slurry: 2 cement to 1 SBR

For a bonding coat under sand and cement render or a cement backing coat, SBR is mixed with cement to make a slurry. The usual mix is about 2 parts cement to 1 part SBR, stirred to a thick, paint-like mix.

It is brushed well into the wall. Then comes the important bit: the render goes on while the slurry is still tacky. The wet render and the tacky slurry bond together. If the slurry dries hard first, it can leave a smooth, sealed face that the render does not grip, so it must be done again.

We use an SBR slurry on things like:

• smooth concrete and engineering brick;
• old, sound render being rendered over;
• pebbledash that is sound and being covered over, alongside the system’s own base coat;
• patch repairs where new render meets old.

Our guide to rendering over pebbledash explains when covering over is a good idea.

SBR as a suction primer: 1 to 4 with water

On a very thirsty wall, SBR can be mixed with water and brushed on to slow down suction. A common mix is about 1 part SBR to 4 parts water. It soaks in and partly seals the surface, so the next coat does not dry too fast. The next coat usually goes on while this is still tacky too.

PVA: useful inside, never under render

PVA is the white glue most people know. Watered down, it is often used inside to control suction or bind dust before a gypsum finish. It can help on some jobs.

But PVA has one big weakness: it softens again when it gets wet. That makes it the wrong choice:

• under any cement render, inside or out;
• on any outside wall;
• in places that get wet, such as behind showers.

We see render and plaster that has come away in sheets because it was put on over PVA in the wrong place. If you are not sure what a previous builder used, tell us and we will check.

Bonding primers for plaster

For gypsum plaster on smooth or painted surfaces, there are ready-made bonding primers. Many are pink or blue and have fine grit in them. Rolled or brushed on, they dry to a rough, sandpaper-like surface that plaster grips well. They are often a better choice than PVA for:

• painted walls and ceilings being plastered;
• Artex and other textured coatings (after checking for asbestos);
• smooth concrete ceilings and lintels.

Our plastering over Artex page covers textured ceilings, and our guide to plastering coats explains which plasters suit which walls.

Primers in render systems

Modern render systems, including K-Rend monocouche and silicone render, come with their own primers and base coats. A silicone top coat, for example, usually needs the maker’s primer over the base coat first. We use the primers the system asks for, rather than swapping in something else, so the system works as it should. See our silicone render and K-Rend monocouche pages.

The other rule: strength

Primers are only part of the story. With render, every coat should be no stronger than the one below, and render should never bridge the damp proof course. A strong primer under a weak render, or a hard render on soft old brick, can still fail. Our guide to render and the damp proof course covers the bottom of the wall.

Quick summary

• SBR slurry, 2 cement to 1 SBR: bonding coat under cement render on smooth or difficult surfaces. Render on while tacky.
• SBR 1:4 with water: suction primer on thirsty walls.
• PVA: inside only, under gypsum, never under render or outside.
• Gritted bonding primer: painted and smooth surfaces under gypsum plaster.
• System primers: whatever the render maker asks for.

For bigger jobs, see our plastering repairs and render repairs pages, or our K-Rend in Birmingham and Solihull page for a full render job.

Not sure what your wall needs before plaster or render?

We work Monday to Friday 7am to 5pm and Saturday 7am to 1pm, and we are closed on Sundays. RCR Connections Ltd is Birmingham based, covering the whole of the Midlands. You can see the towns we cover on our plastering areas and rendering areas pages, and finished jobs on our projects page.

More guides: Plastering coats explained · Render beads and stop ends · Rendering over pebbledash