A plaster is not a colour applied to a wall: it is a system, and it is the binder that decides what the wall will be able to do for the next thirty years. Here is what each family can do, on which background — and the rule that governs all the others.
A plaster is three coats
The trade distinguishes three, and each does a job the other two cannot.
The scratch coat (gobetis) is the bonding coat: one to five millimetres, thrown on, deliberately rough. Its job is not to straighten the wall, it is to make everything that follows hold.
The base coat does most of the work — flatness, thickness, protection against rain. It is applied in one or more passes.
The finishing coat, roughly five to eight millimetres by hand, gives the appearance: the colour, the texture, the surface treatment.
The French reference standard for mortar renders, NF DTU 26.1, sets the expected thicknesses: around 20 to 25 millimetres on carefully built masonry, 25 to 30 on ordinary masonry, with a minimum of 15 millimetres at any point. An industrial “one-coat render” is something else: a single, thinner pass designed for new, even backgrounds.
The rule that governs all the others
If you remember only one thing from this article: each coat must be softer and leaner than the one it covers.
The standard puts it plainly — the binder content and strength of the finishing coat must stay below those of the base coat. And the whole system must stay below the strength of the background.
Why this matters so much: a plaster harder than its background does not move with it. It cracks, comes away in patches and traps water behind it. The wall then deteriorates faster than if it had been left bare. Most damaged old facades do not suffer from a faulty material — they suffer from a render that is too strong.
The binder families
Air lime
It hardens by absorbing carbon dioxide from the air — a slow carbonation, measured in weeks and months rather than hours. Its standard designation, CL 90, CL 80 or CL 70, indicates its minimum lime content.
It is the most flexible and the most vapour-permeable of the mineral families. It can be repaired, topped up, and forgives movement in the background. Its domain: interiors, finishes, limewash, stucco — and exteriors when combined with a hydraulic lime or a pozzolanic additive.
Natural hydraulic lime
It is burnt from a limestone that naturally contains clay, which allows it to set with water within a few hours, before continuing to harden by carbonation. Its designation — NHL 2, NHL 3.5 or NHL 5 — corresponds to its compressive strength at twenty-eight days.
The higher the hydraulicity, the greater the hardness, and the lower the vapour permeability. It is a trade-off, not a quality scale: on old buildings, NHL 2 or NHL 3.5 is the right choice, and NHL 5 is usually too rigid.
Clay (raw earth)
Its binder is raw clay, combined with sand and gravel, and with fibres — plant, animal or synthetic — that limit cracking. It does not set chemically: it dries. That is what makes it reversible: a clay plaster can be re-wetted, reworked and recycled.
Its recognised qualities are the regulation of temperature and humidity and moisture transfer, as well as good airtightness. Its natural domain is indoors; outdoors, it requires a structure protected from driving rain — that is, a generous roof overhang and a protected plinth.
Clay falls outside the scope of DTU 26.1. It is covered by professional rules and good-practice guides specific to the sector: texts written by practitioners, which can be applied contractually.
Gypsum plaster
Quick-setting, vapour-permeable, pleasant to work with, but sensitive to water: its domain is dry interiors. A mix of gypsum and air lime is one of the mortars covered by the standard.
Cement
Hard, fast, impermeable. Those three qualities make it an excellent binder on a modern background — and a poor choice on an old wall.
An old wall has no damp-proof course: it constantly exchanges moisture with the air. Sealed under a cement render, it can no longer evaporate through its outer face. The water comes out elsewhere, at the foot of the wall or indoors; salts crystallise under the surface; and the render eventually blisters, taking part of the facing with it.
The position of the associations that protect historic buildings is unambiguous: on these walls, any water-repellent render or pointing must be avoided, and protection must come from breathable plasters.
Which plaster for which wall
| Background and situation | Sound choice | Avoid |
|---|---|---|
| Old stone or brick wall, exterior | Natural hydraulic lime NHL 2 or NHL 3.5, or air lime with additive | Cement, NHL 5, any water-repellent product |
| Old wall, interior | Air lime, clay, gypsum plaster | Cement, film-forming coatings |
| Earth wall — rammed earth, cob, wattle and daub | Clay, or a very soft air lime | Rich hydraulic lime, cement |
| Modern masonry (concrete block, brick), exterior | Hydraulic lime, or industrial one-coat render | — |
| Concrete | Cement mortar, one-coat render | — |
| Plasterboard, interior | Skim coat, clay or air lime finish | — |
| Plinth exposed to splashback | Richer mortar that is still breathable | Water-repellent render on old buildings |
Finishes, which are not a detail
The same lime gives very different results depending on the surface treatment: limewash, acid-washed finish, polished and burnished stucco, tadelakt, sgraffito, distemper. The texture, the depth and the way light catches the surface are decided here, not in the choice of binder.
This is also when pigments come in. Coloured through the material rather than applied on the surface, they age by developing a patina instead of flaking — that is the fundamental difference between a mineral colour and a paint.
How the choice is made in practice
Three questions, in this order.
What is the background? Its nature, its hardness, how damp it is. You do not choose a plaster from a catalogue: you respond to a wall.
What is the exposure? A facade battered by rain and an interior partition wall call for neither the same family nor the same thickness.
What does the wall need to do? On old buildings, staying breathable is almost always the right answer — and it rules out part of the shelf straight away.
And a fourth, which is not a question but a method: make a sample panel. A square metre on the wall itself, left to dry, tells you more than any technical data sheet. On a serious job, it is not up for discussion.
Let’s talk about your project
Choosing a plaster starts with understanding the wall that will receive it: how it is built, what it has already been given, and where its moisture escapes. The binder comes next, and follows from the rest.
Tell us about your wall — its construction, its exposure, its history. We will tell you which family of plaster it calls for, and why. We visit to see the background and provide a detailed, free, no-obligation quote.
