Can I use Polyfilla over a expanding foam?

[HarrySullivan]

Crack from Window to corner of ceilingBroken air vent that is no longer needed

I have large cracks in gyprock that I have had since moving into the house 15 years ago.

The cracks are nearly 2cm at their widest and travel from the top of a window frame to the corner of the ceiling in a zig zag line.

The length of the cracks are about 40cm in length.

I also have broken air vents that are no longer needed and would like to fill them in as well.

I would like to squirt expander foam into the void behind the cracks and then apply a gap filler like polyfilla to create a smooth finish to paint over.

Would the expanding foam be hard enough to support the use of a gap filler over the top?

Thanks

[JacobZ]

Hi @HarrySullivan,

Welcome to the Bunnings Workshop community, it is fantastic to have you with us.

Expanding foam has a smooth, non-porous, and slightly flexible surface, which makes it difficult for plaster to form a strong mechanical bond. Even if it adheres at first, differences in movement between the rigid plaster and the flexible foam can cause the plaster to crack, flake, or fall away over time. If you were just looking for a short-term fix, it might work for a while, but with any movement, the plaster would likely crack and fail.

I would instead recommend using the method shown in How To Repair A Large Hole In Plasterboard. This will give you a much more stable repair that will last better in the long term.

Let me know what you think and if you have further questions, please don't hesitate to ask.

Jacob