- Varieties of plaster coatings
- Cement mortar for plaster
- What surfaces can be plastered
- When and why beacons are needed
- Substrate preparation by spraying
- Wall throwing technique
- Plaster grout
- Drying and post-processing
Not every home craftsman decides to plaster the walls because of the apparent complexity of the work. We hasten to reassure you: special skills are needed to complete large volumes of work in a short time. Everyone is capable of mastering the basics of plastering with due care and patience..
Varieties of plaster coatings
Plaster itself rarely acts as a finishing wall. The main purpose of its application is to level the surface both in the general plane and with the elimination of local defects, for example, seams and protrusions of brickwork. Following this principle, several types of plaster are distinguished:
- Rough wiping (spraying) is used to improve the adhesion between the outer finishing and the supporting layer of the wall. In this way, surfaces are prepared for finishing, fixed with glue or cement mortar. A typical example is preparation for tiling.
- Ground fill without smoothing is used as a special case of the previous type, when there is a need to level significant curvatures of the wall plane or straighten the geometry of the room.
- Fine plaster (cover) forms a perfectly flat surface with a minimum of pores and defects. This method of rough finishing can be completed with wallpapering or puttying for painting..
In general, all three types of plastering work are carried out sequentially in the given order until the desired surface characteristics are obtained. There are also differences due to changes in the formulation of the plaster. Depending on the required characteristics of the finished surface, the plaster can have increased strength and hardness, hydrophobicity or low thermal conductivity. However, even with special requirements, the application technique does not differ from the base one, only the composition of the solution changes.
Cement mortar for plaster
The material that is applied to the walls to create a plaster coating is ordinary sand concrete with a strength grade from M50 to M200. Since the plaster layer does not perform a direct load-bearing function, the strength requirements are imposed to resist external mechanical influences and plaster tearing under its own weight..
In the classic version, the plastering solution is prepared from washed river or rock sand with a grain size of no more than 1.2 mm for grouting and no more than 2.5 mm for spraying and the main leveling layer. The binder for the mortar is Portland cement of grade 200 or 300, for extra strong plaster – grade 400. The content of cement binder cannot be less than 70 kg per 1 ton of dry sand, but it is usually added in a ratio of 1: 5 – 1: 8 to the filler. Of course, the higher the grade of cement, the lower its content in the plaster solution..
The second option for the preparation of a plaster solution is with partial (from 40%) or complete replacement of the cement binder with hydrated lime or fluff. Milk of lime must contain at least 30% lime by weight, without unqualified inclusions. The main purpose of using lime is to entrain air to reduce the density of the plaster. Accordingly, the consumption of the binder is reduced, and the plaster itself has a lower density and less loads the load-bearing system of the building..
The solutions are prepared on a water basis, the liquid content is normalized based on considerations of ensuring the required mobility of the mixture and its tendency to stratification. The addition of water is carried out after mixing the dry filler and the binder, or by introducing completely dissolved cement into dry sand, followed by bringing the mixture to the desired consistency. The density of the solution for different layers of plaster is different: if the consistency of liquid sour cream is required for wiping the base, then the base layer is thrown in with a thicker paste, in which the marks from pressing with a finger are not filled for at least 2-3 minutes.
What surfaces can be plastered
A wide range of substrates is suitable for plastering work: brickwork, cinder block or porous blocks, concrete grade no higher than M600, as well as prefabricated wooden structures. In general, the suitability of the base for plastering is determined by the adhesion that provides a sufficiently high peel strength.
Surfaces made of heavy concrete are sanded before plastering to increase porosity and primed with deep penetrating compounds that do not form a film. Before plastering, wood cladding of sleeper or frame walls is covered with shingles or steel plaster mesh to improve the quality of adhesion to the base.
The mesh can also be used to bond the plaster layer itself. In this case, it is stuffed after the initial wiping before installing the beacons. Basically, mesh reinforcement is required for plaster thickness from 25 to 40 mm. An additional argument in favor of strengthening can be the high mobility of the base, for example, under unfavorable seismic conditions..
When and why beacons are needed
Linear plaster beacons are necessary in cases where high flatness requirements are imposed on the plaster surface. Beacons help to exclude the formation of irregularities of more than 2 mm / m, in other cases, alignment with a tie bar is sufficient.
Installation of beacons is carried out on small bumps made of ordinary plaster solution with a small addition of stucco to accelerate the setting. To install each beacon, a small portion of the solution is mixed, which is applied to the wall pointwise along a vertical line with an indentation of 50–70 cm. After waiting 2-3 minutes, until the solution thickens a little, a metal lath of the beacon is applied to the wall and immersed in lumps of solution until it coincides with the calculated wall plane.
The most convenient way of alignment is to bring the top of all beacons under a common lacing, align them vertically with a rack bubble level, and then align with the rest of the beacons with a long rule. The distance between the beacons should be such that, with the available length of the rule, it rests horizontally on at least two adjacent beacons. If, after installing all the beacons, local irregularities are noted, the strips can be easily torn off, and then glued again with a new portion of the solution.
Substrate preparation by spraying
The primary spraying is performed with a cement mortar of a liquid consistency. The mixture is rubbed against the wall with a layer about 2–3 mm thick, using a wide spatula or trowel. The remaining sagging can not be removed immediately, but wait for the preliminary setting of the composition.
The main purpose of the plastering preparation is to fill all the pores and depressions on the surface of the substrate. Therefore, the plaster is first carefully and with effort rubbed in, scraping the base with a metal edge, and then the mixture is stretched in a more or less uniform layer. The latter is necessary to prevent drying out of the preparation surface before applying the next layer..
Wall throwing technique
It is possible to apply the leveling layer 10-14 hours after wiping. For leveling without beacons with a layer of up to 10 mm, it is allowed to apply the solution to the wall with a spatula, grater or trowel, that is, by simply spreading thin layers one after another with control according to the rule rail. For a thicker layer, use a plastering bucket.
The sides of the bucket have a special shape that allows the mixture to slip easily when thrown. The movement should take place practically in a horizontal plane with a slight (up to 20?) Tilt of the bucket towards the wall. In this case, the plaster knocks into a tight lump and flies off the bucket with a slight toss up.
The collision of the mixture with the wall should occur with a sufficiently large force, therefore the throw is performed with a smooth acceleration without swing and sharp braking. At the same time, at the moment of collision, the solution is tightly rammed and gets rid of air pores. Plastering on the wall is carried out in sections of 1-1.5 m2 so that the adhering lumps protrude above the plane of the installation of the beacons of the order of 7-10 mm. After filling one area, the excess mixture is removed with the beveled part of the rule and mixed with fresh solution. The protruding hillocks are cut off with the pointed side of the rail quite simply, but in addition to moving from the bottom up, longitudinal vibrations are also made as a rule.
Drying of the main layer before grouting can be either complete or partial within 2–3 days. After the mixture has set, the beacons are pulled out, and the formed grooves are filled with the same solution that was used when throwing.
The formed surface can have numerous defects, cracks and depressions, but at the same time, the cut planes of the bumps form the basis for working with a polyurethane trowel of the appropriate size. The solution is first pressed into the grooves in small portions, and then rubbed over the surface in a circular motion without applying an additional layer. Due to the more liquid consistency of the wiping solution, it is mixed with crumbling aggregate particles, so the surface of the plaster becomes solid and monolithic.
Drying and post-processing
Plastered surfaces are ready for finishing after a full exposure period: mortars on a cement binder dry within 28 days, on a lime binder – within 20 days. During the first half of the established period, the surface should not be allowed to dry out, accompanied by the formation of a web of cracks due to uneven hydration. To eliminate such phenomena, high humidity is maintained in the room: containers with water are installed, and the plaster itself is periodically sprinkled with water using a broom or a hand-held spray gun.
The plaster can be treated with a deep penetrating reinforcing primer prior to application of the final finish. Before painting, it is also possible to wipe with a gypsum putty of a rare consistency to fill the pores and reduce paint consumption.