Use of bonding coats and primers before lime plaster

Is a bonding coat or primer is necessary before applying lime plaster. Below are options and the advantages / disadvantages worth considering.

Beeck and Lime green Plaster Primers

Beeck and Lime Green Plaster Primer are viable options worth considering for lime renders or plasters. These primers will ensure breathability of lime plaster or render, and provide a permanent, alkaline-resistant bond, for challenging high or low-absorbency backgrounds.

Beeck Plaster Primer - With a grippy slurry-like grain texture. This mineral primer will even out differing absorption and texture defects and improve adhesion on smooth substrates with no key, high or low absorbency backgrounds or organically modified surfaces. Ideal as a primer for Ironstone finishing plaster and Adaptavate Smooth finishing plaster.

Solo Plaster Primer – Silic8 Solo Primer a mineral based plaster primer designed to control high suction backgrounds whilst still remaining vapour open. Can also be used to consolidate problematic surfaces and substrates. The primer’s gritted surface provides a key for a new suitable plaster to bond to. Ideal as a primer for Solo one coat plaster.

Polyvinyl alcohol – PVA

PVA is generally unsuitable for use with lime. It’s water-soluble and can emulsify over time, eventually causing plaster to de-bond. It’s also film-forming, so it is not considered a breathable approach.

Styrene Butadiene Rubber – SBR

SBR is a more chemically stable glue that can be used instead of PVA in specific applications. However, like other options on this list, it is not considered breathable as it is film-forming.

Using Bluegrit, Redgrit or Rendaid

Paint-on polymer-based primer coats are generally unsuitable for use with lime work. Applying onto plasterboard, where a lime plaster aesthetic is required to match the other areas of a room and this is generally ok.  But such primers should be avoided on solid walls as they are film-forming, which means they create a barrier preventing breathability or the ability for moisture to move or pass in and out of the building fabric. 

Lime Slurry

When applying render or plaster onto challenging surfaces such as high-suction brickwork, a slurry coat of lime on the wall can help to reduce the suction and increase the bond to the background.

When mixing a slurry, we advise aiming for thin consistency. If the slurry is too thick, it risks reducing the bond strength, as the slurry will want to shrink behind the render or plaster. 

Once your slurry is prepared to the correct consistency, brush it onto the wall with a churn brush. The slurry should be allowed to pull in briefly but not dry. Then, proceed with laying the lime render straight onto this, a couple of square meters at a time maximum on the high-suction background.

If the slurry is allowed to dry before the render is laid into it, the opposite effect can occur, reducing the bond with the background. So, keep a close eye on your slurry after it’s applied to the wall.

For Ironstone plasters and renders we recommend the Ironstone Bonding coat. A breathable ready mixed natural hydraulic lime render and plaster with a coarse texture

suitable as a slurry coat. Ironstone Bonding coat is primarily designed to aid application of further coats of render or plaster basecoat, or cornerstone insulating render on high or very low suction substrates.

For lime green plasters and renders we recommend Lime green stipple coat a lime preparation coat with excellent adhesion to difficult historic structures. Suitable for both high and low suction masonry backgrounds. Provides extra adhesion for further coats of Lime Green Duro or Ultra.