Check of substrate

Simple & safe to the goal

  1. How do you check the strength/load-bearing capacity of a render?

    1. Organic decorative coatings must be removed beforehand. Testing in dry and damp condition provides more reliability. In the case of multi-layer constructions, differences in strength can be determined with a wire brush.

  2. 2. Check the strength by scoring or screwing with a screwdriver. If it does not penetrate or penetrates only a little, the old render is load-bearing enough.

  3. 3. If the screwdriver penetrates easily or deep, the old render is not load-bearing enough. It must be removed.

  4. How do you check an old render for hollow spots?

    1. The entire surface must be checked, especially in the area of cracks. The scaffolding must be in place for this work. Particularly critical checks must be carried out on the west side.


  5. 2. The render surface is run off with wire or a thin metal rod. Alternatively, it can be tapped with a hammer. A dark sound appears in the area of hollow spots.

  6. 3. Hollow areas must be chipped off. If there are too many cavities, it is usually more cost-effective to remove the whole render.

  7. How to recognize the condition of a decorative top coating?

    1. If coatings (paints or renders) are not to be removed during renovation, the old plaster/render basecoat (organic or mineral) must be tested for adhesion and compatibility.

  8. 2. If coatings (paints or renders) are not to be removed during renovation, the old plaster/render basecoat (organic or mineral) must be checked for adhesion and compatibility.

  9. 3. Lime-cement plasters/renders show no reaction to flame exposure.

  10. How do you check the adhesion of an organic decorative top coating?

    1. If coatings (paints or renders) are not to be removed during renovation, their adhesion to the old plaster/render basecoat must be checked.

  11. 2. In the case of paints on rough substrates or thick-layer organic decorative coatings, their adhesion can be tested with the help of a knife or spatula.

  12. 3. In case of smooth surfaces, carry out a grid test:

    - Cut surface of 10 x 10 cm into squares of 2 x 2 mm.
    - Brush off in diagonal direction
    - If more than 80 % remains adherent, the coating is bonding.

  13. How to check the pull-off strength of new renders/mortars on old paints or renders?

    1. Well-adhering old paints or renders can lose their adhesion to the substrate/old render (saponification) in the short or long term after being coated with a lime-cement render (reinforcing mortar). A pull-off strength test can protect against serious consequences (see picture).

  14. 2. Coat an area of approx. 50 x 50 cm with a reinforcing mortar (e.g. weber.therm 300) approx. 6 mm thick. Embed the woven mesh weber.therm 310 in the center of the mortar layer (at least 3 mm overlap) and let it hang out approx. 50 cm on the underside. Cover the surface with a foil during hardening time (approx. 7 days).

  15. 3. The adhesion is sufficient when tearing off within the reinforcing mortar. Adhesion is insufficient when tearing off the coating or when tearing off the coating from the old render/substrate.

  16. How do you determine the porosity of a substrate?

    1. The surface is wetted with the help of a spray or water bottle.

    Case a) Water is soaked quickly, hardly runs off = highly absorbent.
    Case b) Water is slightly soaked, runs off = absorbent.


  17. 2.
    Case c) Water is hardly soaked, runs off quickly = poorly absorbent
    Case d) Water pearls off = non-absorbent

  18. 3. The necessity

    - of the use of primers that reduce the substrate absorption
    - of pre-wetting the substrate
    - of the use of special mortars with improved adhesion results from the determination of the absorption behavior. 


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