When renovating existing buildings, plastering on old plaster is particularly problematic. Reinforcement fabric, which is applied to the old plaster before new plastering, can help. You can find out here when you can attach reinforcement mesh to old plaster and when not.
Interior plaster or exterior plaster and other special features
First of all, you also need to distinguish between reinforcement mesh on external plaster and on the inside. Especially with external plastering, new plastering can be even more demanding. Here we therefore deal with the application of reinforcement mesh to interior plaster. You also need to know the condition of the masonry and plaster:
- Also read - Apply reinforcement mesh to interior plaster
- Also read - reinforcement mesh on external plaster
- Also read - plastering reinforcement mesh
- there are cracks in the plaster; if so, solid or growing cracks
- there may be coatings such as paint applied
- Absorbency of the old plaster
- Compatibility of the old plaster with the reinforcement plaster
Is the plaster still stable??
First of all, the plaster must be knocked off with a hammer to see whether it is still firm enough everywhere. Cavities and plaster that is no longer really adherent can be found quickly. Then the test is carried out with a screwdriver. Scratch the plaster everywhere. If the screwdriver does not or hardly penetrates, the old plaster can remain on the wall because it is still strong enough.
The absorbency of the old plaster
Water is then applied to determine how quickly the water is absorbed. It takes experience to correctly interpret the absorbency. Basically, the faster the water is sucked up, the sooner you need a deep foundation. If most of the water runs off, you need a primer (€ 19.50 at Amazon *) .
Load capacity of other coatings such as paint
Now the load-bearing capacity of other coatings such as paints has to be determined. To do this, score small 2 x 2 mm squares in 10 x 10 cm squares. If the layers of paint still adhere to at least 80 percent of the squares, the substrate is considered to be sufficiently stable for a new plaster.
Fixed and growing cracks in the old plaster
In the case of existing cracks, you need to know whether the cracks are static or growing. With growing cracks, knock off the plaster sufficiently around the crack. Then fill the area with the appropriate mortar (€ 7.79 at Amazon *) and fiberglass reinforcements.
In the case of non-migrating cracks, knock off the area around the cracks and fill in with repair mortar. Remember that different masonry, for example brick and sand-lime brick, have different thermal expansion. Then cracks are inevitable in many plasters. Only plasters with a high relaxation behavior can help here.
Different reinforcement fabrics for old plaster
Now there are also different types of reinforcement. Newer reinforcement mats are worked into the surface of the reinforcement plaster and removed again after it has hardened. Other reinforcement fabrics remain in the plaster. So you need to know exactly which system you are using. If the reinforcement mesh remains in the reinforcement plaster, it should be applied in a thickness of up to 6 mm. Then the actual exposed plaster follows.Tips & Tricks Make sure that the different plasters, old plaster and old coatings can be used together (e.g. emulsion or silicate paints). Otherwise the plaster will not achieve the desired load-bearing capacity.