Here are some tips on how to proceed to plaster the surface of a wall or ceiling, and in particular on how to properly prepare the mortar.
The plaster is nothing more than a mortar consisting of a fine sand, with grains no greater than two millimeters in diameter, mixed with additives and with a binder that incorporates the whole and allows it to harden. The type of binder used for the mortar also serves to define the type of plaster, which can be based on lime, lime-cement, cement-lime, gypsum or clay.
The sand can be calcareous or siliceous and can have natural origin, therefore of fluvial origin, or be derived from grinding; the choice of the type of sand and binder to be used in the mortar mix depends on the type of plaster and its intended use, which can be internal or external. The additives added to the mixture also vary according to the characteristics that the plaster must have; these additives can be plastics, cellulose, micro silica, starch and more.
Furthermore, premixed plasters in bags are available on the market, to which only water must be added; each premix has a specific composition for a particular type of application, for which it is possible to find internal plaster , external plaster , skim coat, and so forth, depending on the requirements of the case.
How plaster is done
In the event that the plaster has to be prepared starting from the individual components, these must be mixed in the right quantities. The proportion also varies according to the type of mortar, for interiors or exteriors; the plaster for interiors, for example, is prepared with a mixture of ten parts of sand, three of lime and one of cement, while in the plaster for exteriors, any additives must also be taken into account. Both the binders, therefore lime and cement, as well as the sand and additives, must first be mixed dry in order to obtain a homogeneous mixture with a grain size not exceeding 1.5 or 2 millimeters at the most.
The water must be added and mixed a little at a time, until the mortar reaches the right consistency; the best way to check it is to pull the back of the shovel on the dough with a sawing motion, thus forming ridges. If the latter hold their shape without flaking, crumbling or falling off, then the consistency of the mortar is good.
Once prepared, the plaster has a workability time of about an hour in optimal climatic conditions; if the weather is particularly hot and dry, on the other hand, then the installation time is reduced to about 30 minutes. Consequently, it is important to always prepare a quantity of plaster that can be disposed of within these time limits, otherwise there is the risk of losing part of it due to hardening.
The consistency of the mortar can also vary according to the layer to be applied; the base coat, for example, also called rough coat, is usually prepared with a coarser sand with a more liquid consistency, in order to improve the adhesion of the subsequent layers.
How to plaster
After having prepared the mortar, making sure it is well mixed and of the right consistency, it is possible to begin plastering the walls by applying the first layer; l ‘ plastering , in fact, is nothing more than a coating that is applied on the rough wall in successive layers, so as to level the surface and prepare it for painting. The first layer is the most consistent and is called a rough coat, or base plaster; this is the only layer that is applied to the wall unevenly, so as to cover about 60% -80% of the surface of the masonry.
The rough coat should be applied using a trowel, taking a small amount at a time and throwing it on the wall, or using a special compressed air sprayer; once applied it should not be touched, but left to dry as it is in order to leave a rough surface which will act as an adhesion bridge between the rough masonry and the subsequent layers of plaster.
The second layer is called curl, or more simply second hand, and is characterized by a finer grain size; the arriccio constitutes the real skeleton of the entire plaster coating, in fact its task is to uniform the surface of the masonry, to act as a protective barrier and, at the same time, as a load-bearing structure for the finishing plaster , or for glue and tiles.
Furthermore, before applying the arriccio on the rough coat, it is necessary to nail some wooden strips to the wall, at a distance of about 90 centimeters from each other; these strips will serve as guides for the straight edge; after having applied the arriccio with the trowel, in fact, passing the straightedge it will be possible to level the layer evenly.
The last layer of plaster is called a veil, or even a civil plaster , and has the function of protection and finishing; often this layer is made directly with the smoothing putty, or with smoothing plasters. The civil plaster is applied only with a trowel, after which it is rectified where it is needed with the help of belt sanders , a wall sander or sandpaper.
How to plaster the ceilings
The steps listed above are the same as when applying plaster to the ceiling. Usually this type of plastering must be performed on ceilings made with pignatte and joists, or with joists and large slabs, and before proceeding, scaffolding must be prepared, so as to be able to work near the ceiling.
The latter must first be wet, after which the rough coat can be applied following the splashing technique; considering the horizontal position of the surface and the technique used, it is important to wear protective goggles and overalls, so that you can work without worrying about getting dirty.
The rough coat can also be applied with the so-called American technique , that is, instead of splashing it with a trowel, it is put on the trowel and then applied to the ceiling, spreading it with alternating movements, so as to make it already well leveled and uniform.