Do you need to prime British Gypsym Thistle walls?

[Groucho]

After removing old paint from unsealed walls, we decided to skim-coat them with British Gypsum Thistle Multi-finish. This is a hard plater regularly used in the UK, where a watered down emulsion (ceiling paint) is applied as a base coat, before top coat. 

Is there any reason to use a primer in this scenario? The walls are not chalky, they are solid. There is no need for oil-based primer so deciding between water-based primer and diluted ceiling paint. Both pass the tape test after 24 hours. 

[JacobZ]

Hi @Groucho,

Thank you for your question and welcome to the Bunnings workshop community, it is fantastic to have you with us.

Priming will seal the surface to prevent the paint from soaking into the plaster, meaning you won't need to use as much paint. It creates a uniform base for the paint which will ensure a consistent finish. Finally, it will improve the adhesion of the paint, giving it more longevity. 

There are definitely benefits to priming the wall, but unfortunately, I am not aware of a water-based primer that is suitable for this purpose. 

Using a watered-down paint emulsion might give you some benefit, but it is hard to say how effective it would be long term. If you have tested it in a small area and are happy with the result, this might be the most cost effective method.

 I have spoken to Dulux who have advised using Dulux PRECISION White Sealer Binder, which is an oil-based primer. Unfortunately, I'm not aware of a water-based primer that the manufacturer will guarantee for this use.

If you are against using an oil-based primer, I'd suggest following the advice of the plaster's manufacturer as they will have the best information regarding their product.

If you have any further questions, please don't hesitate to reach out.

Jacob