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Step 1 - Pick the wall
The staking of the wall is one of the preparatory works for the realization of the plaster. It is necessary in a few cases: - When the previous coating crumbles, no longer holds on the wall. In this case, we will use an ice ax to strip the joints until we find a healthy and relatively solid base. Be careful, in any case do not picket beyond 2 to 3 centimeters back from the face of the stones, even if the coating seems friable. This is completely normal for an old plaster. By staying 2/3 centimeters back from the face of the stones, your new coating will come to grip with the old one and will have an ample grip. - When all or part of the coating protrudes from the face of the stones. In our example, we are dealing with a simply masonry wall, which has never been plastered. However, the mortar is thickened in places. It is these surpluses that we are going to stitch out so that they do not level with the level of the final coating, with the risk of creating irregularities (by color, grain size, etc.). After picketing, dust the wall with a brush to remove residue.
Step 2 - Protect the stones (optional)
As with any messy activity, prevention is easier than cure! Depending on the nature of the stones, lime can taint their surface. It is not completely unsightly, but it is more pleasant all the same when the stones remain intact and can proudly display their original colors! To protect them, you can brush them with milk using a large brush. This method is slightly restrictive, since it must be applied regularly, several times while waiting for complete drying between each application, but gives very good results. If the damage is done, after drying the wall, rub the stones with a cloth soaked in alcohol vinegar. However, if you are experienced, you can get started by skipping this step!
Step 3 - Water the wall
The humidification of the wall is the other essential preparatory work for the realization of a plaster. From the start of the application and until complete setting several days or weeks later, the wall will indeed "drink" the water from the fresh coating that is applied to it. If this phenomenon is too great or too abrupt, the new plaster will be difficult to work at the time of application and will weaken on drying. In general, it will be unsightly and not very durable. For watering, two options which will depend on the level of soiling that the environment of the wall allows: the water jet or the sprayer. A first watering is ideally done the day before, a second before the start of the application of the coating. In dry and hot weather, it may be useful to humidify the wall at regular intervals, to avoid setting too quickly.
Step 4 - Prepare the plaster
There, even if there are general rules, in practice, a certain latitude in the composition of the mixture is possible, depending on the desired effect, the support, the variety of sands available ... Let us stick to the outline of this "step by step" by advising a mixture respecting the following proportions: - 5 volumes of sand 0/4 - 2.5 volumes of air lime type CL90 - Water
In our example we use a slightly different recipe for our interior coating. - 1 volume of quarry sand - 3 volumes of local red sandstone - 2 volumes of CL90 lime - 0.4 volume of local mineral earth This allows our new coating to look like a brother to the old one. The mixture is prepared in a concrete mixer by adding the ingredients to the stop (try to wedge the lime between 2 buckets of sand to avoid making an unpleasant cloud to breathe). Add the water gradually to better dose it. The ready-to-apply coating must have a consistency that is both thick and flexible: if you take it from the nose of your trowel, it should not leak. In the case of small quantities, the coating can be mixed by hand in a trough. Too much water will promote the crazing during drying, conversely an insufficient supply will promote the chalking of your coating.
Step 5 - Apply the coating
First, it is necessary to determine a bias ... and stick to it! The question facing you is that of the treatment of stones intended to remain visible, the others being covered with plaster. The choice is made taking into account two factors: - The aesthetics of the stones: are they small, irregular ... or on the contrary quite large and at least partly cut? - The flatness of the wall, and that of the faces of the stones
This one, we're going to cover it
That one, well we decide to leave the stones more or less visible. A "stone-look" coating is by no means a traditional technique. If you look carefully at an old building whose facade has not been restored, it is easy to notice that the stones only appear when the plaster begins to disintegrate over time (rain, changes in temperature… ). And if we look more closely, we notice that only certain stones emerge from the plaster - the larger ones and those whose faces are straight, which offer less grip to the plaster -, and that the joints between they are not hollow. In fact, you really have to imagine the wall at its origin, with the stones completely covered with plaster, then imagine it for years after having received day after day the "caresses" of the rain, the sun ... What l 'we see then, it is the subtlety with which this erosion was made. As much as possible, a new coating which aims to be respectful of traditional techniques will seek to cover the stones so as to keep visible only the faces of the most beautiful and / or most salient (we make "as if" the new coating that we apply had already benefited from the patina for years). You should know that the coating - which traditionally applies in several layers - constitutes a protective layer for the building. Our elders did not gargle as we do in front of the exposed stone. Most of the frames were made of stone then, and it was above all for them to ensure their longevity by covering them with a protective layer. Once these explanations are received, we look at our wall taking into account the two factors described above. If the stones are small and irregular - which is generally the case with our wall -, we cover them widely so that only the most notable appear. In the same concern to be respectful of old techniques, we will not try to mark the joints between the stones (hollow joints) and we will ensure the overall flatness of the wall. Of course, some will be tempted to say that it is all a matter of taste. Maybe ... but not only! It is also a matter of knowledge and respect for old buildings, knowing that once the eye is warned ... the taste is in turn able to evolve! Let us clarify one more point: if a stone plaster seen with very dug joints can be considered "pretty", if it is applied outdoors, it will be much less durable and protective than a covering plaster. At the time of energy savings, this is also a factor to take into account!
Shall we summarize? The coating should be applied in the plane of the wall, keeping visible only the most notable stones. The coating will be projected - if you have mastered the gesture - or applied directly with the back of the spatula. There is no real rule when it comes to starting the app. The simplest is perhaps however to start from the bottom, so that by progressing upwards, the humidity of the coating that is applied migrates towards the bottom with the effect of slowing down drying (which is preferable).
Scratch with the edge of the trowel by scraping the edge of the stones to remove any excess.
Step 6 - Tighten the plaster
As the application and drying, we monitor the appearance of microcracks that we just "tighten" if necessary so as to strengthen the adhesion of the coating. For this, the simplest is to use the tip of the trowel, but other tools - spatula, trowel ... -, can also do the trick. These microcracks can be due to a too rapid drying time, to a too fatty mixture, to a too thick coating layer… They are normally superficial. However, you might as well track them down to recreate the cohesion of the coating and make it more aesthetic.
Step 7 - Brush the plaster
This step is one that reveals the beauty of the work done over the previous hours. By means of a quilting or metallic brush, we remove the excess coating present on the faces of the stones or in relation to the plane of the wall. The choice of the type of brush is made according to the hardness of the coating, that is to say according to whether it has already largely set or not. The best is to test.
If when you try to apply the quackgrass brush, the coating sticks between the bristles, then it's too early. At the right time for the intervention, work again in accordance with the wall plan, using gentle and varied gestures. It is better to pass several times over part of the wall, brushing it in one direction then in another, than to scratch violently in just a few passes.
Step 8 - Dust the plaster
This final touch-up can be done one to a few days after the application of the coating, using a flexible brush. We then try to finish closing the plaster and drop the surface grains of sand. After this stage, if the plaster has been properly made, it will age without bringing more dust or deposits than a wall with a conventional coating.
Here, we covered the wall, leaving only the large stones to appear, which is the closest to an old plaster.
Another form of rendering, less covered and more economical, to keep the stones visible.
The stones are flush with the wall, suggesting that the patina of time has done its job.