How to prepare rendered internal walls for a square set finish?

Question

Hi,

We have are going to rip out all our cornice to achieve a square set look. It is a brick rendered unit. Can we use plaster filler over the render to smooth it out/fill cracks before painting or will this cause a reaction? What would the process need to be for it to not cause the paint to bubble and flake if we used plaster filler instead to rendering it. Thanks

EricL · Answer

Hello @nika

When considering a finish over a rendered wall, it is generally not recommended to apply gypsum plaster directly onto cement render, particularly where there is any risk of moisture. While gypsum plaster can provide a very smooth and attractive finish, it is far more sensitive to moisture than cement-based products. Cement render naturally absorbs and releases moisture, and if that moisture migrates into the gypsum layer, it can cause the plaster to soften, weaken, crack, or eventually detach from the wall surface. In addition, cement render and gypsum plaster expand and contract at different rates as temperatures and humidity levels change, which can lead to cracking and adhesion issues over time.

For the best long-term performance, especially on rendered walls, I would generally recommend staying within a compatible cement-based or polymer-modified finishing system. This approach helps maintain good adhesion, durability, and resistance to moisture-related problems.

If you need further assistance, please let us know.

Eric

nika · Answer

Thanks Eric.

If we wanted to use plaster Instead of render is this also possible? What steps would you recommend?  thanks

EricL · Answer

Hello @nika

Thank you for sharing your question about your rendered wall. It appears that your wall has a fine topcoat render, giving you that fine finish on the surface, I don't recommend using plaster on your rendered wall. Once you have removed the cornice on your ceiling, I suggest using a

https://www.bunnings.com.au/100mm-sharp-edge-paint-scraper_p1670156

to remove any leftover cornice, adhesive and debris on the wall and ceiling. I then suggest following the guide below.

I also recommend looking at this product for your project -

https://www.bunnings.com.au/dunlop-20kg-fine-coat-render_p0760244

.

Here is a handy step-by-step guide:

https://www.youtube.com/watch?v=bX7xMvLZwEk

Please remember to wear personal protection such as gloves, goggles and a mask.

If you need further assistance, please let us know.

Eric