Exterior painting is most often associated with the application of protective and decorative paint coating on facades. Facades are heavily loaded surfaces and their preparation for new painting or renovation requires careful inspection and treatment before applying primer and paint.
1. The preparation of façade surfaces must start after the repair works inside the building have been completed and after the interior surfaces have dried completely. If repair works on the roof structures are also planned, they must also be performed in advance - especially the installation of gutters and downspouts.
2. For all types of external surfaces, it is of particular importance to be dry and free from efflorescence (stains of water-dissolved salts), grease or mold and algae (single-celled algae). Do your best to eliminate the source of moisture. Once the surface is completely dry, carefully clean the efflorescence, greasy stains or mold and algae. Use appropriate chemicals for this purpose.
3. Exposed concrete and masonry must be carefully cleaned with water, under high pressure (eg water jet). If, after this type of treatment, there are still weak or crumbling areas, use a mechanical scraping tool and clean to a solid base. Do not use aggressive percussion methods. Repair the damaged areas with a suitable cement-based repair mortar. Do not use putties or tile adhesives.
- If there are cracks on the surface, expand the cracks to a size of about 2 mm, in a "V" shape. Fill the cracks with a suitable cement mortar.
- Damaged joints in masonry must be cleaned and filled with cement-based mortar.
- If the surfaces are very uneven or rough and a flat surface is required, level the base with lining plaster. For a better finish, a layer of putty can be applied on the dried lining plaster. For exterior surfaces, use cement-based products.
4. Painted surfaces of concrete, masonry or plaster should be checked for peeling or swollen old paint. Scrape the damaged paint to a firm finish. It is recommended that the places with removed paint be repaired with a thin layer of putty.
- In the case of glossy paint coatings, the surface must be sanded with fine sandpaper. Paints do not adhere well to glossy surfaces. Wash the treated surfaces to remove dust and allow to dry.
- In case of suspicion of adhesion of the paint to the substrate, a test on a small area is recommended, as with a model knife, lines are cut in the paint in the form of a grid with a square size of about 2 cm. is left to protrude. Pull the tape sharply to peel it off. If there are no flakes of paint on the back, it can be assumed that the coating has good adhesion.
5. Existing cement plasters should be checked for adhesion to the substrate. This can be most easily determined by tapping a blunt object (such as a wooden hammer handle) on the surface. Robust plaster makes a light and dense sound. Plaster with poor adhesion, emits a specific sound of knocking on the cavity. Weak spots must be scraped to a solid base and repaired.
6. Loose plasters must be stabilized with a deep penetrating primer. Given that crumbly plasters may have slight imperfections of worn material, the primed and dried surface can be treated with a suitable putty.
7. Cracks in old plasters must be repaired. Paints do not have the ability to bridge cracks.
8. Robust structural or relief plasters on cement, silicone, silicate or polymer base must be well cleaned of accumulated dust and dirt, especially in the lower parts of the relief. Sweep the surfaces with a broom or brush with long stiff bristles, then wash with pressurized water.
9. Natural limestone usually only needs impregnation with a colorless product specifically designed for this. This will preserve its natural appearance. If stone surfaces are to be painted, they must be cleaned of crumbling particles, dust, mold or algae. This also applies to cases where the stone will be impregnated.
In practice, many more construction situations can be encountered, but the above activities are the most commonly applied. Thus treated, most exterior surfaces, such as facades, are a good base for laying finishing systems of primer and paint.