Prepare the background for plastering & testing the suction

Building methods and materials are always evolving, with many changes to the way we work and our practices. Modern materials have been developed and manufactured to improve the efficiency of mixing plaster and to ensure compatibility with different backgrounds, simplifying the plastering process. Despite this, plastering surfaces can still break down over time due to age, poor surface preparation or bad workmanship (including poor mixing).

The key to creating a good plaster surface is to identify and prepare the backgrounds beforehand. It is important to understand that not all backgrounds have the same properties. Some will be soft and weak, some will be hard and dense, while some backgrounds could be timber studwork, joists or metal furrings. Plasterboard fixed to old timber surfaces might need to be built out with backing plaster, depending on how uneven the surface is. This is quite common in old buildings that are being refurbished.

Different backgrounds need to be prepared for plastering in different ways. There are several steps you need to follow to make sure the plaster adheres well to the background surface. Before you start, you will need to control the suction and ensure the background has adequate key to ensure adhesion.

Suction Test

A suction test can be carried out by applying water on a solid background to see how much it absorbs. Checking the suction will tell you if the background is dry and porous. You can do this by splashing water onto the background with a brush. The quicker the water is absorbed, the higher the background's suction.

• No suction or low suction when the splashed water stays on the background's surface) will indicate that the background is hard or dense.
• High suction backgrounds (when the splashed water soaks in quickly) will absorb moisture from the plaster mix and might cause it to dry too quickly when applied.