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Making a Mechanical Key on your Surface

Last modified: 02.07.2023
A bucket of Topcoat Primer placed on a construction site surface in front of a modern building; used as a base layer to ensure strong adhesion and durability for exterior wall coatings.

In the context of plastering, the term "mechanical key" refers to the roughened surface or texture which allows a new layer of plaster to adhere properly to an existing surface. Essentially, it's the grip or bonding point that the new plaster layer uses to cling to.

The mechanical key is essentially the textured surface that is needed for a new layer of plaster or renders to properly stick to an existing or base layer. This texture can be achieved in several ways, depending on the nature of the surface that's being worked on.

  1. Scratching the surface: After a base layer of plaster (or render, when working outside) is applied, it can be scratched while still wet to create a rough, textured surface. This is typically done with a special tool called a scarifier or a devil float. These have metal teeth designed to create a uniform pattern of grooves in the plaster.
  2. Physical structures: Some surfaces naturally have a rough texture that provides a good mechanical key. For instance, bare brick or concrete walls usually don't need any additional treatment for the plaster to stick to them. The irregularities in these materials provide ample points for the plaster to grip on.
  3. Using a bonding agent: In some cases, especially for very smooth or low-suction surfaces, a bonding agent may be required. These can be brushed or rolled onto the wall, and they dry to form a rough, gritty surface that provides a strong mechanical key for the plaster. Examples include PVA (polyvinyl acetate) and SBR (styrene-butadiene) based bonding agents.
scratched surface to create a mechanical key

Suction refers to the rate at which a substrate (the surface that's being plastered) absorbs water from the plaster. This is crucial because if the plaster loses water too quickly, it can dry out and harden before it's had a chance to bond properly with the substrate. Therefore, resulting in a weak and potentially unstable finish.

  1. High suction surfaces: Common substrates such as brick and porous stone are highly absorbent and can suck the moisture out of the plaster very quickly. This can be counteracted by damping down the wall with water before plastering, which slows down the rate of suction. Alternatively, a suction control layer, sometimes called a "spatter dash" coat, can be applied. This is a rough layer of mortar that is thrown or splashed onto the wall, and it not only provides a good mechanical key but also helps regulate the rate of suction.
  2. Low suction surfaces: On the other hand, substrates like dense concrete or surfaces that have been painted or treated with a waterproofing agent have very low suction. The lack of water absorption can lead to poor adhesion of the plaster. These surfaces often need to be roughened. Alternatively, a special bonding agent may be used to ensure the plaster sticks properly. It's also possible to use a plaster that's specially formulated for low-suction surfaces. This has additives that improve its bonding properties.
  1. Brick: Traditional bricks are made from fired clay, a material that naturally absorbs water. This is why brick walls need to be managed carefully when plastering. The high suction rate means that water from the plaster can be absorbed rapidly, leading to premature drying. If not managed, it could lead to a weak bond between the plaster and the brick and might cause cracks as the plaster dries out. Using techniques such as pre-wetting or applying a mist coat of bonding agent can help slow the suction rate and allow the plaster to bond more effectively.
  2. Aerated concrete blocks: These are made with a mix of cement, lime, water, and an expansion agent that creates many small air pockets within the block. This gives them a high insulating value but also makes them very absorbent. This high suction can cause similar problems as with brick, and similar methods can be used to control it.
  3. Porous stone: Natural stone substrates like limestone or sandstone has a high level of suction due to their porous nature. The irregularities in the surface of these stones can provide a good mechanical key. However, the high suction rate can cause issues with premature drying, similar to brick and aerated concrete.
  • Brick
  • Aerated concrete block
  • Porous stone
 
  1. Dense concrete blocks: Dense concrete blocks are made from cement and aggregate, without the addition of an expansion agent. This makes them more solid and less porous than aerated blocks. They have a slower suction rate, meaning they absorb water more slowly. This can make it harder for the plaster to bond with the surface and may require a bonding agent or a roughened surface to provide a mechanical key.
  2. Rendered or previously painted surfaces: Any surface that has been previously painted or rendered is effectively sealed. Therefore, its porosity is reduced and hence suction is low. For these substrates, a mechanical key would need to be created, either through physical abrasion or through the application of a bonding agent. It's also recommended to use a plaster that's designed for low-suction surfaces.
  3. Metal and glass: These materials are not porous and thus do not absorb water. For these substrates, the plaster cannot rely on suction for bonding at all. A strong mechanical key would need to be created, usually through the use of a specialised primer or bonding agent that's designed to adhere to these materials.
  4. Plasterboard (drywall): Plasterboard has a paper surface that has some level of absorption. However, this is not as much as bricks or aerated concrete blocks. Plasterboard that has been treated for moisture resistance has an even lower suction rate. When plastering onto plasterboard, a plaster that's formulated for low-suction substrates is often used. In some cases, the surface may be lightly roughened or a bonding agent may be applied to aid adhesion.

Our primers are specially designed to create a mechanical key on substrates. This can be for the application of a basecoat or adhesive. Alternatively, our primers are also applied prior to a render layer which ensures the correct adhesion.

EWI-310 Universal Primer is used to prepare the walls before the application of our adhesives. It improves the adhesion of the adhesive to the substrate, facilitating application and reducing groundwork absorbency.

EWI Pro Top Coat Primer is designed to be used in conjunction with any of our silicone renders. There are several reasons for using the Top Coat Primer. Firstly, it aids adhesion between the render and the basecoat. Secondly, it limits the absorption of the basecoat layer, ensuring the render cures properly – this is especially important during the summer months, giving the installer time to get a finish on the render. Finally, the Top Coat Primer can be tinted in the same colour as the render being used, ensuring that there is no basecoat bleed-through.

EWI-302 Deep Penetrating Primer is a substrate preparation product, used for priming new build external walls prior to the application of renders or mortars.

EWI-301 Water-Based Substrate Primer is designed for priming various different absorptive substrates including ordinary and aerated concrete, walls made of bricks, blocks and structural clay tiles, plaster coats, gypsum panels and gypsum plaster boards.