Machine Applied Plaster - Cement-Lime Plaster

The machine adds water at a steady rate, so the mix has the same consistency across the whole wall. On larger areas this makes the work faster, and the surface comes out more even than with plaster mixed by hand in a bucket. The usual sequence looks like this:

1. Checking and preparing the background. The plasterer checks the masonry, removes dust and mortar snots, fills larger joints and, depending on the background, applies a primer or a bonding coat.
2. Fixing the beads. Plaster depth beads go on the walls to set the plane. Angle beads go on corners and window reveals to protect the edges.
3. Setting up the machine. The water supply is adjusted so the mix is neither runny nor too stiff.
4. Spraying. The plaster goes on in strips, usually from the bottom up, as one continuous layer. The thickness is usually 10 to 20 mm, depending on how straight the masonry is and on the manufacturer's instructions.
5. Ruling off. A long aluminium straightedge is drawn across the beads. Surplus material is taken off and low spots are filled while the plaster is still fresh.
6. Finishing the surface. Once the plaster has stiffened a little, it is trimmed flat and then rubbed up with a sponge or felt float. This gives a fine surface that can be painted or skimmed with a thin finishing coat.
7. Curing. The plaster dries naturally, with gentle ventilation and no aggressive heating.

Most plaster problems start before the machine is even switched on. The background must be sound, clean and free of salts. Loose old layers, oil-based paint and leftover tile adhesive have to come off, otherwise the new plaster will not hold.

Every material soaks up water differently:

• Fired clay brick is moderately absorbent, and a primer that evens out the suction is usually enough
• Aerated concrete blocks are very absorbent, so they need a primer and a plaster the manufacturer explicitly recommends for them. Otherwise the plaster dries too fast and cracks
• Smooth concrete (ceilings, lintels, cast walls) takes up almost no water, so it needs a sand-filled bonding coat that creates a rough key

All services in the walls must be finished before plastering. Cables and back boxes must be fitted and checked by a qualified electrician, and the boxes should sit flush with the future plaster surface. It makes sense to pressure-test water and heating pipes before plastering, because fixing a leak under finished plaster is far more work. Chases left after the services go in should be filled in advance and, if they are wide, covered with mesh.

Plastering an interior on its own does not usually need a permit. If the job also includes building work, such as new openings in walls, check the local building regulations with your local building authority. In a flat, let the building management or owners' association know about noisy work and any work affecting shared services.

Cement-lime plaster combines the strength of cement with the breathability of lime. It is hard, resists knocks and abrasion and handles moisture better than gypsum. That is why it is chosen for bathrooms, kitchens, hallways, cellars, garages, boiler rooms and walls that will be tiled.

Gypsum plaster is finer, and after finishing it is often smooth enough to paint directly. It suits dry living spaces. Manufacturers allow it in bathrooms only under certain conditions, for example with good ventilation and a liquid waterproofing membrane under the tiles. It is not suitable for cellars or garages.

Indoors, machine-applied cement-lime plaster is usually done as a one-coat system: the body and the surface are formed in one go and finished with a felt float. If you want a smoother wall, for example under a light colour with low-angle light from a window, a thin skim coat or fine filler is applied once the plaster has cured.

How demanding the job is, and therefore what it costs, depends mainly on the state of the wall before plastering:

• Straight masonry in a new build is the quickest to plaster because a thinner layer is enough
• Uneven walls in older houses need a thicker layer, sometimes applied in two passes, and more material
• Hacking off old plaster and carrying out the rubble adds extra work
• Many windows, reveals, corners and recesses mean many beads and a lot of hand finishing
• High ceilings and stairwells need scaffolding
• Access for the machine also matters: it needs water, power and space for the bags, and in a flat on an upper floor the hoses have a long way to run

If you can, plaster the whole house or flat in one go. The machine then only has to be set up and cleaned once.

• Plastering onto a dirty or loose background. The plaster then sounds hollow when tapped and can come away.
• The wrong mix consistency. A runny mix loses strength and leaves patchy areas. A stiff mix is hard to rule off and leaves a rough surface.
• Ruling off too late. Waves and hollows stay in the wall.
• Bad timing of the float finish. Worked too early, the surface drags and closes up. Worked too late, it can no longer be flattened.
• Drying too fast. Draughts, direct sun or heating on full cause shrinkage cracks. Never plaster in frost. Most manufacturers give a minimum temperature of roughly +5 °C.
• Painting or tiling onto wet plaster. The paint may stain or peel. A common rule of thumb is about one day of drying per millimetre of thickness, but the manufacturer's figure is what counts.

For the handover, bring a long straightedge or spirit level and a torch. If you shine the light along the wall from the side, you will see unevenness much more clearly. Agree the allowed flatness tolerance in the contract beforehand. Tiling, especially large-format tiles, needs tighter tolerances. Check:

• Flatness of the walls with the straightedge, vertically, horizontally and across the whole area
• Plumb corners and reveals, with straight, undamaged beaded edges
• A continuous surface with no patchy areas, cracks, bleeding or tool marks
• Tapping the walls to make sure nothing sounds hollow
• Back boxes and outlets: clear, clean and flush with the plaster
• Clean windows, frames, floors and pipework with no plaster splashes
• Instructions for drying and ventilation, and the date from which painting or tiling can start