In an old house, the choice of paint decides whether the wall can breathe or traps water. This guide sets out what the LNEC (Portugal’s national civil engineering laboratory) says about lime walls, how lime paint behaves, how to prepare the surface and when lime is not the answer. For the general comparison of paint types, see emulsion, acrylic, enamel and others.
Updated 4 November 2026. General information, not a substitute for a technician’s assessment.
The essentials
- Old walls with lime render let water in and out. The LNEC recommends mineral paints, which do not form a film, and warns that emulsion and acrylic paints work badly on these surfaces.
- Lime paint does not cure damp. If the cause is not treated, the problem returns. See the types of damp.
- It is an alkaline product (pH 13 on the Com-Cal data sheet). Wear gloves, goggles and a mask.
- It does not go over emulsion paint or low-porosity surfaces, according to the Com-Cal data sheet.
- Limewash needs an experienced applicator. The LNEC gives this as a reason to prefer silicate paints in some cases.
Why lime suits old walls
An old wall is thick, porous and has no capillary break. The LNEC explains that it takes in some water but dries quickly towards the outside, which is how such walls lasted centuries. If it is covered with cement or a low-permeability paint, the water stays trapped, may rise higher in the wall and the damage spreads to other areas.
The values below are indicative, from the LNEC (2006). Sd is the equivalent air layer thickness: the higher it is, the more the coating holds back vapour.
| Coating | Sd (m) |
|---|---|
| Lime render | 0.07 |
| Cement render | 0.12 |
| Lime render with lime paint | 0.08 |
| Lime render with silicate paint | 0.10 |
| Acrylic paint | 0.30 |
| Membrane-type paint | 1.50 |
In a 2002 study the LNEC also measured plain limewash: its Sd (0.15 m) was very close to unpainted lime render (0.14 m). It concluded that limewashes do not hinder the drying of the wall, while silicate paints slow it moderately. The study covers facades.
What lime paint is
Limewash (caiação) is painting with milk of lime (lime in water), plain or with mineral pigments. The LNEC recommends air lime in putty form, well slaked, rather than powder, for better coverage, adhesion and durability. It is applied in thin coats, on a damp wall, over a rough render.
Ready-to-use lime paints also exist. Three data sheets we read:
| Product | Data sheet figures |
|---|---|
| Com-Cal, interior (edition of 26/02/2021) | CL90 air lime putty; µ = 15; VOC < 1 g/L; 6 to 8 m² per litre per coat; recoat after 4 to 6 h; at least two coats |
| Com-Cal, exterior (edition of 26/04/2024) | 3 to 4 m² per litre in two coats; apply between 5 and 30 °C |
| Caoval, by Isaval (page read on 4/11/2026) | Do not dilute; 7 to 10 m² per litre per coat; recoat in 3 to 4 h; full cure in 15 to 20 days |
Coverage figures vary, so check the data sheet of the product you will buy. For litres, use the calculator and the guide how much paint do I need.
When it works
- Old lime render, plaster (estuque) or gypsum, in good, dry condition.
- Walls that you want to stay permeable to vapour.
The Com-Cal sheet says it suits absorbent mineral surfaces such as gypsum, plaster or lime mortar, plasterboard and cement.
When it does not work
- Over emulsion or acrylic paint. The Com-Cal sheet says not to apply it on water-repellent or low-porosity surfaces (varnish, plastic, wood, metal). If the wall carries resin paint, lime does not bond. See the LNEC warning in the paint types guide.
- To hide salt, mould or water stains. The cause must be treated first: see salt on walls and mould.
- In heavily polluted cities. The LNEC writes that in large urban centres limewash is “practically unviable” and silicate paints last longer.
- When nobody can apply it well. The LNEC names the lack of experienced applicators as a reason to prefer silicates.
On durability, the LNEC says a limewash can stay in good condition for several years. Repainting every year is mainly to keep it white.
Preparation, step by step
- Treat the cause of the damp before painting.
- Identify the substrate. Lime render, cement render and gypsum behave differently. In an old or heritage building, ask a technician.
- Remove anything loose. The Com-Cal sheet asks for a firm, dry, clean surface, free of dust, grease, efflorescence and micro-organisms. Repair cracks and holes and let them dry.
- New lime render: the LNEC (ISEL, 2017) gives one month of drying before painting, with a paint that is very permeable to vapour, preferably mineral.
- Uneven absorption: Com-Cal asks for a water-based primer, to avoid colour differences.
- Do a colour test in a discreet spot. The sheet warns that different batches may vary slightly.
- Apply at least two coats, respecting the interval on the sheet.
To paint an already limewashed wall with emulsion paint, the Duquebel sheet (Fixador de Cal, revision of 20/06/2022) says to brush off all poorly bonded lime with a wire brush and use the product, diluted to 10%, as a sealer.
Safety
Lime is alkaline and irritates skin, eyes and airways. Com-Cal lists gloves, goggles and a respirator, washing splashes off at once with water and protecting surfaces that are not being painted.
What we could not confirm
- How to tell lime render from cement render at home. We found no official source for a simple test. Ask a technician.
- Washability of lime paint: the sheets we read do not state it.
- Commercial claims such as “purifies the air” or “natural antimicrobial”: these are Com-Cal’s, and we did not verify them.
- Prices of lime paint and of application: we did not read any.
- The Robbialac (Vixalit) sheet: the page refused access.
- Standards. Com-Cal cites DIN 18363 and Directive 2004/42/EC. We read DIN 18363 only through the LNEC, and did not read the Directive.
- Whether the LNEC study, made on facades, applies fully to interiors.
Sources (read on 4 November 2026)
- LNEC, Maria do Rosário Veiga and Martha Tavares, Características das paredes antigas. Requisitos dos revestimentos por pintura (Characteristics of old walls. Requirements for paint coatings), APTETI, October 2002. Main text, sections 1 to 6.
- LNEC, Maria do Rosário Veiga, Intervenções em revestimentos antigos: conservar, substituir ou… destruir (Interventions on old renders: conserve, replace or destroy), 2006. Sections 1 to 6.
- LNEC, Maria do Rosário Veiga, Rebocos antigos: como intervir? (Old renders: how to intervene?), ISEL course, October 2017. Slides on defects and paint application.
- Com-Cal, Pintura de cal, interior, data sheet in Spanish, edition of 26/02/2021.
- Com-Cal, Ficha técnica, Tinta de cal para exteriores, edition of 26/04/2024.
- Isaval, Caoval, lime-based paint, product page. The PDF data sheet was not opened.
- Duquebel, Fixador de Cal, data sheet, revision 7, 20/06/2022.