Fine Lime-Cement Render (Feinputz)

The mix contains lime, cement and very fine sand. Lime keeps the coat slightly flexible and vapour-permeable, so the wall can "breathe". Cement adds strength. The grain is fine, usually around 0.5 mm, and the layer is thin, typically a few millimetres according to the manufacturer. Feinputz does not level out large unevenness. That is the job of the base coat underneath.

Do not confuse it with decorative plasterwork. Cornices, ceiling roses and ornaments are a separate specialist job. Here the topic is a plain, fine top coat across the whole wall or ceiling.

Interior finish coat is used in living rooms, hallways, kitchens and, with good ventilation, bathrooms. What matters most is a smooth, uniform look and good adhesion for paint. In a bathroom, areas that get splashed need tiles or a waterproofing membrane instead, because the finish coat itself is not waterproof.

Exterior finish coat has to cope with rain, frost and sun. Use a mix that the manufacturer clearly labels for outdoor use. Façade work is more demanding: you need scaffolding, you have to watch the weather, and the details around windows, sills and the plinth take care. If you live in a block of flats, talk to the building management or owners' association before touching the façade. For larger changes to how a façade looks, check with your local building authority whether you need to notify them or apply for permission.

1. Check the substrate: the base coat must be firm, dry, dust-free and properly cured. A rough rule of thumb is about one day per millimetre of base coat thickness, but the manufacturer's data sheet has the final word.
2. On old render, tap off loose areas, remove old paint and grease, and repair cracks.
3. Dampen or prime the surface so it does not suck the water out of the fresh coat too quickly.
4. Cover windows, floors and fittings. Outdoors, put up and secure the scaffolding.
5. Mix the material as the bag instructs, always using the same amount of water per bag.
6. Apply a thin layer with a steel trowel, by hand or by machine.
7. Once the surface has firmed up slightly, work it over with a sponge or felt float to get an even finish.
8. Protect the fresh coat from direct sun, draughts, frost and rain until it has cured.

Hand application suits smaller areas, older houses and rooms with lots of corners and openings. Machine application is faster and makes sense on large, flat walls in new builds. Either way, the final finish is always floated by hand.

Feinputz belongs on mineral base coats, meaning lime-cement or lime render. Brickwork with a base coat is the simplest case. Aerated concrete blocks are very absorbent, so priming is a must. Where different materials meet, for example brick and a concrete ring beam, a glass-fibre reinforcing mesh goes into the base coat so that a crack does not appear later. Smooth concrete first needs a bonding primer, otherwise the render will not grip well.

Do not put lime-cement finish coat on gypsum plaster. The two materials do not work together. If the wall is gypsum, a different skim product is used.

The condition of the substrate matters most. A flat, clean, well-cured base coat means quick progress. An old, crumbly or cracked surface means tapping off, repairs, priming and sometimes mesh. Other factors include:

• The size and shape of the area, since a large flat wall goes faster than small rooms with many corners
• Height, because ceilings and tall rooms need a mobile scaffold tower
• The number of windows, doors, reveals and corner beads
• Outdoors, scaffolding, protective netting and waiting for suitable weather
• Whether a furnished home has to be thoroughly covered and cleaned up afterwards

In practice, the most frequent problem is applying the finish coat to a base coat that has not cured long enough. The base is still shrinking, and fine hairline cracks show up on the surface. Another classic is drying too fast: direct sun, draughts or heating turned up full straight after rendering. The cement does not cure properly and the surface ends up dusty or cracked. Mixes are usually meant to be worked roughly between +5 and +25 °C. The exact range is on the manufacturer's data sheet.

Other trouble comes from skipping the primer on an absorbent wall, a layer that is too thick and cracks, or one that is too thin so the unevenness shows through. Painting too early is also common. The wall must dry out first, which usually takes several weeks. Otherwise you get patchy shading or peeling paint. On façades, damage happens when rendering is done just before rain or frost.

A small indoor area, such as a repaired wall in one room, is something you can manage yourself with a bit of patience. Leave façades and any work at height to a professional with proper scaffolding.

1. Clear the room and cover the floor and windows with plastic sheeting.
2. Tap off loose areas and brush the dust off the wall.
3. Fill larger holes and cracks with base coat beforehand and let it cure.
4. Prime the wall or dampen it evenly.
5. Mix a small batch exactly as the instructions say.
6. Apply a thin layer with a steel trowel, working from top to bottom.
7. Wait until the surface firms up, then float it with a sponge float in circular movements.
8. Keep windows closed against draughts, keep the heating low and let the wall dry slowly.
9. Paint only once the wall is completely dry.

Look at the wall in daylight and also with a lamp shining at a low angle along the surface. That shows up every wave.

• The surface is uniform, with no patches, scratches or clumps of grain
• There are no cracks on the wall or in the corners
• Running your palm over it leaves no dust and nothing crumbles
• A long straightedge shows no noticeable dips or waves beyond the agreed tolerance
• Corners and reveals are straight and crisp
• Junctions at ceilings, windows, doors and switches are clean
• Outdoors, the details at sills and the plinth are carefully finished
• The product and manufacturer are recorded in the handover notes, together with any defects still to be put right