Hello everyone,
A garden wall built from concrete formwork blocks has already been coated with the base render (cold cement render) in the required thickness according to the manufacturer's instructions, including a mesh reinforcement.
Now the topcoat, made of lime-cement render, is to be applied because we want a smooth surface without texture, which will then be painted.
The base render on the different walls has been in place for 1.5 years on the oldest wall and since last autumn on the newest one.
It was forgotten to roughen the base render with a scratch coat after it had partially dried, so now it has a smooth surface that is somewhat shiny in places. A primer has already been applied, but after rolling over it, the surface still feels smooth and not rough enough. As a test, I tried to apply some leftover render from another project onto the wall, but it did not stick and immediately fell off.
What kind of surface preparation should I do now to ensure the topcoat will properly adhere? I had considered applying a bonding agent, but the manufacturer does not approve its use for exterior applications.
Thank you very much for your help.
Best regards
A garden wall built from concrete formwork blocks has already been coated with the base render (cold cement render) in the required thickness according to the manufacturer's instructions, including a mesh reinforcement.
Now the topcoat, made of lime-cement render, is to be applied because we want a smooth surface without texture, which will then be painted.
The base render on the different walls has been in place for 1.5 years on the oldest wall and since last autumn on the newest one.
It was forgotten to roughen the base render with a scratch coat after it had partially dried, so now it has a smooth surface that is somewhat shiny in places. A primer has already been applied, but after rolling over it, the surface still feels smooth and not rough enough. As a test, I tried to apply some leftover render from another project onto the wall, but it did not stick and immediately fell off.
What kind of surface preparation should I do now to ensure the topcoat will properly adhere? I had considered applying a bonding agent, but the manufacturer does not approve its use for exterior applications.
Thank you very much for your help.
Best regards
Hi,
With such a smooth, compacted surface without a roughened texture, adhesion becomes tricky—especially outdoors.
If the base plaster already has a “glassy” finish, a standard primer won’t help much. Concrete bonding primer is usually not approved for exterior use (as you correctly mentioned) because it’s not UV- or frost-resistant.
What you can do:
Roughen the surface mechanically, for example with a spiked scraper, plaster milling tool, or simply by tapping with a hammer and spatula. It’s enough to break up the top, compacted layer.
Then apply a suitable bonding primer designed for exterior use (for instance, silicate-based), which adheres well to smooth mineral substrates.
Only after that apply the topcoat plaster, and don’t wait too long so the primer remains tacky.
It’s also important to check if the bonding primer you already used was suitable for exterior applications. Otherwise, it’s better to remove it before it peels off later.
Regards,
Chris
With such a smooth, compacted surface without a roughened texture, adhesion becomes tricky—especially outdoors.
If the base plaster already has a “glassy” finish, a standard primer won’t help much. Concrete bonding primer is usually not approved for exterior use (as you correctly mentioned) because it’s not UV- or frost-resistant.
What you can do:
Roughen the surface mechanically, for example with a spiked scraper, plaster milling tool, or simply by tapping with a hammer and spatula. It’s enough to break up the top, compacted layer.
Then apply a suitable bonding primer designed for exterior use (for instance, silicate-based), which adheres well to smooth mineral substrates.
Only after that apply the topcoat plaster, and don’t wait too long so the primer remains tacky.
It’s also important to check if the bonding primer you already used was suitable for exterior applications. Otherwise, it’s better to remove it before it peels off later.
Regards,
Chris
Thank you very much for your reply. I had almost expected that I would have to take this step. I will rent a scarifier and remove the top layer by about 1mm (0.04 inches). That way, the surface will be smooth. I believe the primer was universal, meaning suitable for both interior and exterior use, but to be sure, I will double-check before applying it to avoid any problems.
A simple universal deep primer should be sufficient, right? Or what is needed for the rough, scarified surface of the lime-cement render to ensure that the lime-cement finishing render adheres properly?
A simple universal deep primer should be sufficient, right? Or what is needed for the rough, scarified surface of the lime-cement render to ensure that the lime-cement finishing render adheres properly?
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