White powder on the wall, paint that swells and flakes off near the skirting, plaster that turns to sand when you touch it: in Portugal people call this salitre (literally “saltpetre”). Specialists call it efflorescence. This guide explains what it is, where it comes from, why painting over it does not work, and what paint and mortar manufacturers say must be done before you paint again.
Updated on 3 November 2026. General information; it does not replace an assessment by a qualified professional.
The essentials
- Salitre is salt dissolved in water that gets into the wall. When the water evaporates, the salt stays behind and crystallises (LNEC, Portugal’s national civil engineering laboratory).
- No water, no salt damage. The first step is always to find and stop the source of moisture.
- Crystals on the surface (efflorescence) look bad but do not break the material. Crystals that form inside the pores (subflorescence) are what crumble plaster and push paint off (LNEC).
- An anti-salt primer slows down efflorescence, the data sheets say. It does not replace stopping the water.
- Where the plaster has already decayed, CIN says to remove the affected plaster, re-plaster and wait about 1 month before painting.
What salitre is, in plain words
Stone, brick and mortar are full of pores. Water enters them carrying dissolved salts. When the wall dries, the water leaves as vapour and the salt stays behind as crystals.
LNEC researchers Teresa Diaz Gonçalves and José Delgado Rodrigues describe two cases:
- Efflorescence: the salt crystallises on the surface. You see it as white powder or crust. According to LNEC it causes no mechanical damage, but it spoils the look of the wall and affects how healthy the room is.
- Subflorescence: the salt crystallises inside the pores, just under the surface. The crystals build up stress that can break the material apart. This is where crumbling plaster, blistering paint and fine cracks come from.
The same paper adds that salts can, on their own, cause or worsen damp problems.
Where the water and the salt come from
The LNEC authors explain why the problem is so common in older houses:
- old materials are very porous and soak up water easily;
- walls sit directly on the ground, so water rising from the soil by capillary action is common;
- salts and moisture from different sources build up over the years, especially during periods when the roof leaked or the plaster was in poor condition.
In recent buildings, moisture usually comes from construction defects. Even so, because diagnosis is hard, repairs are often repeated.
To work out which source you have, read Damp walls: three types and how to tell them apart. If the water comes from the flat above, see Leak from upstairs: who pays.
How to recognise it
| What you see | What it usually is |
|---|---|
| White powder or crystals that rub off by hand | Efflorescence on the surface |
| Paint blisters full of powder, crumbling plaster | Salt crystallising behind the paint |
| Damp or stained band rising from the floor, with an uneven edge | Possible rising damp from the ground |
| Patches that darken on humid days and fade in dry weather | Salts drawing moisture from the air |
| Black or green velvety spots | Mould, not salt. See the mould guide |
The table is a pointer, not a diagnosis. Salt content is measured in a laboratory, as LNEC does in its tests.
Why painting over it does not work
The LNEC paper describes what happens when a coating that lets neither water nor vapour through (it calls these “sealing” renders) is applied to a salt-laden wall:
- Water keeps arriving and looks for another way out. It may move sideways into nearby parts of the wall, which then take the damage.
- When the coating cracks, as it eventually does, water and salt escape through that point. Crystals concentrate there and the damaged area keeps growing.
So a fresh coat of emulsion, or a “waterproof” paint, put on a wall with salt and water still coming in is likely to blister again. The salt has not gone: it is sitting underneath, waiting for a way out.
What the manufacturers say
We read three technical data sheets. All of them start from the same point: the surface must be dry, sound and clean.
| Product | What it is for, per the data sheet | How it is applied |
|---|---|---|
| CIN Anti-Salitre 17-680 (revised June 2023) | Water-based coating that slows the appearance of efflorescence; preventive treatment for interior or exterior walls | 1 coat, ready to use; 10 to 14 m²/L; recoat after about 16 h |
| Robbialac Primário Anti-Salitre 020-0402 (11/03/2024) | Water-based primer to prevent salt and slow efflorescence; moisture-sealing primer for plaster | 2 coats undiluted; 6 to 10 m²/L; topcoat 1 to 2 days later |
| Robbialac Barreira Anti-Salitre 148-2006 (06/02/2021) | Hydraulic-lime mortar forming a barrier against salt coming from damp walls | Layer at least 1 cm thick, up to 0.5 to 1 m above the damp line |
Three warnings in those sheets are worth reading closely:
- CIN says to remove areas with efflorescence and rotten plaster over a wider area than the affected one, re-plaster and let the new plaster cure, which takes about 1 month. It adds that the causes of the damp must be removed to stop the salt coming back.
- Robbialac says its primer is not suitable for walls with condensation damp.
- In Robbialac’s mortar system, the old plaster is stripped back to the masonry up to about 1 metre above the crystallisation zone. On top goes a porous mortar (Restaura Desumidificante), 2 to 3 cm thick, which according to its sheet helps the wall dry out.
Note the verb both primer sheets use for efflorescence: slow down. None of the three says the treatment holds while water keeps coming in.
Steps before you paint
- Find the water. Roof, gutter, pipe, soil banked against the wall, condensation. Without this step, everything else is temporary.
- Stop the water or reduce it as far as you can. For rising damp, get a professional assessment: solutions depend on the wall.
- Remove loose salt and any paint that is not firmly attached. LNEC lists mechanical removal of efflorescence among the control methods.
- Strip decayed plaster, beyond the stained area, as CIN advises.
- Re-plaster and let it cure (about 1 month, per CIN). Do not rush: every sheet we read requires a dry surface.
- Apply the treatment set out in the data sheet of the product you choose, with the stated coats and times.
- Paint with a finish compatible with that primer. To work out how much paint you need, use the calculator.
- Watch the wall through one damp season. If the white powder returns, water is still getting in.
Who to call
A small patch whose cause is already fixed: a painter used to plaster work can handle it. A band rising from the floor on several walls, or one that returns: first someone who can diagnose the damp, then the painter. Ask for the preparation in writing (plaster to remove, area, product, curing time) and see how to compare painting quotes.
What we could not confirm
- Prices for salt treatment or new plaster. We found no official price table. The price depends on how much plaster has to come off, the cause of the damp and the system chosen.
- How long each treatment lasts. The sheets say “slow down”, with no time frame.
- Washing salt off with water. We found no official source that recommends or advises against it indoors. If in doubt, follow the product’s data sheet.
- Systems to stop rising damp (chemical barriers, ventilation at the base of the wall, among others). We did not read enough official sources to compare them.
- The state of your wall. This guide is general. Only a professional on site can confirm the cause.
Sources (read on 3 November 2026)
- LNEC, Teresa Diaz Gonçalves and José Delgado Rodrigues, Argamassas de revestimento para paredes afectadas por cristalização de sais solúveis: influência do substrato (“Renders for walls affected by soluble salt crystallisation: influence of the substrate”), paper for the APFAC 2010 conference (sections 1.1 to 1.4: origin of salts, efflorescence and subflorescence, how renders work, sealing renders, drying).
- Pedro Puim, Teresa Diaz Gonçalves and Vânia Brito (LNEC), Controlo e prevenção de anomalias devidas à cristalização de sais solúveis em edifícios antigos, PATORREB 2012 (abstract only: list of control methods).
- CIN, Anti-Salitre 17-680 technical bulletin, revised June 2023 (description, surface preparation, application, coverage, drying).
- Tintas Robbialac, Primário Anti-Salitre 020-0402 data sheet, version of 11/03/2024 (description, use, application, coverage, other information).
- Tintas Robbialac, Barreira Anti-Salitre 148-2006 data sheet, version of 06/02/2021 (substrate preparation and application).
- Tintas Robbialac, Restaura Desumidificante 148-2016 data sheet, version of 12/10/2021 (use, thickness, drying).
Products are mentioned as examples of public data sheets. ParedeCerta.pt does not sell or endorse any brand.