What must a substrate be like? How do I prepare the substrate?
In principle, a substrate, whether for paint, plaster or wallpaper, should be solid, dry, evenly absorbent and load-bearing. However, the evenness requirements of the substrate vary.
With plasters, the substrate does not need to be quite as smooth as with paints and wallpapers.
What does solid, dry, evenly absorbent and load-bearing mean and how do I create such a substrate?
Dry: newly applied plasters, levelling compounds and primers should be sufficiently dry. There should be no moisture damage due to penetrating or rising damp.
Firm: The plaster should not sand. You can consolidate a sanding plaster with a water-based deep primer.
Load-bearing: Old glue or binder paints are not stable. These colours have a floury and chalky surface. They are not smudge-proof. If you wipe over them with a moistened finger, traces of colour will remain on your finger. These paints must be washed off or sanded down as the new surfaces, whether paint, wallpaper or plaster, do not adhere well to them. They can flake off and fall off. In the case of wallpaper, the seams often open up or even fall off.
Once most of the glue and binder paints have been removed, the last remnants can be consolidated with a water based deep primer.
The binder paints were often painted over with standard emulsion paints, although this is not professional practice. (This is particularly common in old buildings, which were painted with binder paints until the first half of the 20th century. The first emulsion paints came onto the market after the Second World War).
The first coat may also appear to adhere. However, the colours often come off when the next coat is applied. In this case, the top layer of emulsion paint must first be scraped off and then the glue or binder paint removed.
Evenly absorbent: Unevenly absorbent substrates, e.g. the change between levelling compound and cardboard in the case of plasterboard substrates, can lead to visual problems with the new surfaces. The easiest way to equalise the absorbency of the substrate is with a water based primer.