Why does cement not adhere to plastic

Plastering for beginners - this is how it works with the smooth wall

You have never plastered before, but you don’t want to miss the chance to throw the trowel yourself? You have made quite a difference, because cleaning is one of the more difficult DIY activities. Here you will get to know the basics of plastering.

Table of Contents

The tools you need

Correct plastering stands or falls with the right tool. You will need the following tools:

  • Stainless steel smoothing trowel for applying the plaster
  • Float for smoothing and compacting
  • Puller (trowel) made of stainless steel, with which plaster is spread wide
  • Nail board for roughening the plaster
  • Grooming brush for removing the plaster
  • Felt board for moistening and smoothing the surface
  • Mixing paddle for mixing

Step 1: prepare the subsurface

Careful preparation of the subsurface is even more important than having the right tools. If you slut here, the plaster may not hold up later. Basically, the subsurface must:

  • be free of layers of dust and dirt
  • be absorbent
  • be rough
  • be dry
  • no moisture must rise

In addition, it depends on the material the plaster base is made of. Additional measures are then required to prepare the substrate:

Concrete as a plaster base

A freshly concreted concrete wall needs to dry for about 4 weeks so that the subsurface is reasonably dry. However, concrete is only slightly absorbent, so that the plaster does not adhere as well. This can be improved with a spray grout. More on that later.

Brick as a plaster base

Brick masonry is a grateful plaster base - it is sufficiently absorbent and also very rough. This then leads to good interlocking between the plaster and the substrate. A wide variety of plaster types adhere here without an adhesion promoter.

Sand-lime brick as a plaster base

Sand-lime brick is very smooth and very absorbent. This is not a suitable plaster base. It is therefore always necessary to pre-treat with an adhesive bridge paint or a spray grout.

Pumice concrete as a plaster base

Walls made of pumice concrete are very rough, which is why plasters adhere very well to them. No spray grout is therefore necessary. However, since the absorbency is rather moderate, you should pre-wet the wall before plastering.

Aerated concrete as a plaster base

Aerated concrete is a very highly absorbent material, which means that plaster cannot set properly, as sufficient water must be available for this. In any case, an adhesive bridge coating or a full covering spray coat is necessary

Plaster wooden beams or steel girders

Plaster does not stick to wooden beams or steel girders. You must therefore clad them with a plaster base made of wire mesh or expanded metal before plastering.

Spray grout for better plaster adhesion

With some substrates, as already mentioned, it is necessary to ensure better adhesion of the plaster by means of a spray grout. This is an almost thin-bodied mortar that you have to throw on the wall with a forceful swing using a trowel. Joints and other recesses on the wall must also be filled in this way.

The mortar for the grout must be so fluid that it runs practically completely from the horizontally held trowel. Mix the mortar again and again, as the ingredients settle very easily. It is also important to pre-wet the wall so that the moisture required for setting is not removed from the mortar.

If the substrate is very absorbent, a fully covering spray coat is required. If it is weakly absorbent, a not fully covering spray coat is sufficient, the plaster base may still shine through.

In the case of a non-covering spray grout, you should wait at least 12 hours before you start plastering. The mortar must be sufficiently firm. If you pre-inject several millimeters thick in a closed position, you will have to wait correspondingly longer. When the mortar can no longer be wiped off by hand, you can start plastering.

Step 2: plastering

Let's start with a few preliminary remarks: A plaster can consist of one or more layers. An exterior plaster should be applied in two layers with a total thickness of 20 mm, with an interior plaster 15 mm is sufficient. It can be plastered in one or more layers.

In dry weather, plaster made of lime or cement must be kept moist by spraying water. Also protect fresh exterior plaster from wind and strong sunshine. It must not dry out too quickly, as this could lead to cracks, for example. See article plaster cracks.

Danger! Lime, cement and plaster of paris are corrosive and should therefore not get in the eyes. If it happens, rinse the eye out with clear water immediately. Also, make sure that you do not touch the mortar with your hands as much as possible. Therefore wear rubber gloves. Wearing protective goggles can also be helpful - especially when mixing the mortar.

Mixing the plaster mortar

The mixing ratio of the plastering mortar depends on whether you mix the plaster by hand or with a machine. It is the same as when making masonry mortar.

The components of the plastering mortar cannot be mixed so well by hand, so you should choose a lower proportion of sand than when mixing with the machine. Use the smaller value in the table below if you are applying the plastering mortar by hand.

Mixing ratio of plaster mortar

Mortar typeAir / water limeHydraulic limecementsand
Lime mortar

(Use as interior plaster)

113,5 – 4,5

3-4

Lime cement mortar (use as exterior plaster)1.5 or 219-11
Cement mortar (use as base plaster or exterior plaster)Up to 0.52

1

6-8

3-4

Gypsum mortar (use as interior plaster)Processing without sand or as a gypsum sand mortar with 1 to 3 parts of sand
Anhydrite mortar (use as interior plaster)Mixture of 1 part anhydrite and up to 2.5 parts sand

The table shows the mixing ratios in parts of space. On the construction site, you usually measure the parts with a shovel, which is of course not particularly accurate, especially since a shovel filling varies greatly depending on the moisture in the sand.

Mix the mortar components very carefully so that the grains of sand are covered as tightly as possible by the lime or cement paste. You can recognize a good mix by the even color of the mortar.

If you only need to mix small quantities, you can do this quite well by hand, but if you need larger quantities of mortar, it is better to use a mixer.

Here are a few more tips for mixing the individual types of mortar:

  • Mix for making Cement mortar Dry the cement and sand and then add the water. You can do the same with lime mortar when the sand is dry.
  • If the sand is damp, the lime can form lumps. Then mix in Lime mortar first the lime with water and then add the sand.
  • Sprinkle to turn on Gypsum mortar into the mixing water. Mix the mortar after a short period of soaking. Mix only small amounts, as the processing time is only about 15 minutes.

If you don't want to deal with mixing ratios, you can also use ready-made mixtures. They are a little more expensive, but you get a mortar that is evenly mixed in the right proportions. All you have to do is add water and you don't need a mixer.

Special case of plastic plaster

Plastic plasters are made on the basis of emulsion paints with aggregates. They don't have to be touched, they come to the construction site in buckets. You then only have to stir up plastic plaster briefly with a drill with a mixer insert.

A plastic plaster requires a smooth plaster surface as the substrate. The plaster itself determines the look and also protects the wall.

Pull on the plaster with a stainless steel trowel and pull it off to grain thickness with a steeply set trowel. Then rub through with a plastic float. The thickness of the plaster is determined by the grain thickness.

Depending on whether you rub the plastered surface horizontally, vertically or in a circle, different surface structures result. However, don't do this for too long as it will worsen the appearance of the surface. The reason: the plaster is constantly being torn off by the grain.

Concealed

Let's start with the concealed installation. The first layer of plaster is 10 to 15 mm thick. You should choose the strength of the concealed plaster so that it is larger or at least as large as the second layer of plaster. This is a principle of plastering: the strength of the plaster layers should decrease from the inside out or remain the same.

Apply the mortar in one step only so thickly that the plaster adheres well and does not slip off. Also, apply a second layer of plaster first because the first layer is hard enough.

Throw in the mortar

Now it's down to the nitty-gritty, you have to make sure that the mortar stays on the wall. Throw the mortar vigorously from bottom to top on the wall and make sure that the lumps of mortar overlap each other. With a nail board you can roughen the surface a little and improve the adhesion.

If you should try to spread the mortar with a smoothing board, it may work, but: There may be cavities between the plaster and the wall, which could lead to the plaster falling off.

Do not undertake too large areas in one go, but work in sections. The individual area should not be larger than about 2 x 1 m. This is the only way that the fresh mortar can be worked on evenly for the duration of the work.

Peel off plaster

Once you have thrown on the mortar with the trowel, the next step is: you have to level the surface with a long float - the grooming brush. Place the grooming brush on the floor and pull it upwards with side-to-side movements.

Excess mortar will collect on the float. Throw this into recesses in the plaster with a trowel. Do this until the freshly plastered surface is really even.

Rub off the plaster

If the mortar can hardly be pressed in, you can rub off the plaster. To do this, run the float over the plastered surface with light pressure. This makes the plaster even smoother and thicker. If you rub the plaster off too early, the surface will be wavy. If the plaster has become too dry, you can moisten it slightly.

Bottom corners

For a clean floor corner, place the brush upright in the corners and move it back and forth with light pressure. Pick up the excess mortar lying on the floor with the trowel.

Then work on the bottom corner with a float - just like you did with the wall surface.

Tip: Check the plaster's evenness regularly. This works best with a straight batten or board. Place the lath in different directions, it must always lie flat on the plastered surface.

Protect corners with rails

Outside corners are always at risk from you or someone else bumping into them and damaging them. You should therefore absolutely plaster corner plaster rails at the critical points. This does not have to be done under the ceiling, up to a height of about 1.50 m is usually sufficient.

Cut the corner protection profile to the desired length and bend the ends slightly inwards so that they do not penetrate the plaster later.

Place chunks of plaster about 50 cm apart on the corners and press the corner protection rail into it. Align the rail straight with the spirit level. If it is stuck, you can plaster the wall.

Cleaning gauges make work easier

Even experienced plasterers find it difficult to create precisely straight plastered surfaces. So-called cleaning gauges are valuable aids here. These are, for example, slats with a thickness of 1 to 1.5 cm that are attached to the wall with wall hooks. There are also metal plaster strips that can be attached to the wall with plaster of paris.

When attaching the cleaning gauge, use a spirit level to check that it is really vertical. When removing the plaster, you can pull the grooming brush along the gauge and thus have the certainty that the surface will be straight and even.

Plaster a corner

You already know how to protect a corner with the help of rails, now it's a matter of making a straight and sharp edge:

Get a notice board and attach it to the corner so that it protrudes about 1.5 to 2 cm beyond the corner - just as far as you want the plaster to be. It is attached with plaster hooks or steel nails.

Throw the plaster along the board and proceed as you learned in the sections before. The edge of the board serves as a guide.

If the plaster is firm enough, carefully remove the plaster gauge. You then attach these to the plaster that has already been completed in order to plaster the other surface of the corner. Be extremely careful here so as not to damage the fresh plaster.

Now you can plaster the second surface. Once the plaster has set, remove the plaster gauge and repair the impact points with fresh mortar.

Finishing coat

The finishing plaster determines the appearance of the wall; numerous ready-mixed finishing plasters are available for this. They are often treated with dyes or special types of sand, so that a subsequent coat of paint is not necessary.

Plastering is the same as for concealed plastering, but you can use the smoothing board to give the plaster different structures - depending on whether you rub the plaster horizontally, vertically or in a circle. You can also structure the plaster with a trowel, a notched trowel or other tools - just as you find it most beautiful.

Fine plaster mortar as interior plaster

Fine plastering mortar made from ready-mixed dry mortar is often used as the finishing coat for interior walls. You just have to stir it up by adding water with the paddle until it is smooth and creamy. The exact information can be found in the manufacturer's instructions.

Only apply the finishing plaster when the base plaster is firm enough to support the new plaster layer. You can tell by the fact that you can no longer press it in with your thumb. If it is too dry, you can moisten it a little with a brush.

Apply the fine plastering mortar about 2 mm to 3 mm thick with a trowel. To do this, take it in your left hand and use a trowel to put it on the trowel with your right hand. Left-handers can of course do it the other way around.

Now continue working with two hands. Place the trowel at an angle at the bottom of the wall and pull it up with light pressure. Proceed in the same way for the next strips until the entire wall has been provided with a layer of plaster.

Now proceed as you did with the concealed plaster, first peel it off and then rub it off until a smooth surface is created. To rub off, apply the smoothing trowel at a slight angle and guide it over the plaster with semicircular movements.

Repair imperfections in the plaster

In principle, damage in the plaster can be repaired more easily than plastering an entire wall from scratch. The basic rules of plastering technology naturally also apply here.

Before you can start plastering, you must first knock off the loose plaster generously. That can be more than too little, because a damaged plaster causes all sorts of inconveniences, including the formation of mold.

Thoroughly clean the masonry, brush it off and pre-wet it. Now throw the plaster on the patch. You can achieve better adhesion if you mix the mortar with a chemical adhesion promoter.

Once the plaster has dried, you can plaster the patched area. When rubbing off, pay particular attention to the transition between old and new plaster, here clearly visible edges can arise. With a wet brush or brush, you can even out and smooth the transition after rubbing with the smoothing trowel.

Video: plastering the living space

If you want to look at moving pictures rather than reading through everything here, then I have found a video for you here:

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