What is efflorescence (salt deposits) and how to treat it

Efflorescence, often called salt deposits, is the salts left on the surface of a wall when water evaporates. It shows up as a white powder or crystals and makes the paint blister and fall off. To treat it, you need to fix the damp causing it, remove the salts and only then paint.

What efflorescence is

The water inside the wall carries dissolved salts, which come from the wall's own materials or from the ground. When the water reaches the surface and evaporates, the salts stay behind as crystals.

That gives it its typical look: whitish patches, white powder, a rough crust or crumbling plaster. In Portugal it is commonly called salitre.

How to recognise it

white powder or crystals on the wall, which come off with your hand or a brush;
paint that blisters, cracks or peels;
soft plaster that crumbles;
stains that appear mainly on the lower part of the wall;
the wall may be damp on rainy days and show signs of salt on dry days.

Salt deposits are very common in old houses, on ground floors, in basements and on walls built against the ground. Near the sea, as in Cascais and Oeiras, salt in the air can also contribute.

Why it appears

Salt deposits are a sign that water is passing through the wall. The most frequent causes are:

damp rising from the ground up through the wall;
rain leaking in through façades, roofs or terraces;
leaking pipes;
soil banked up against the wall, with no drainage;
lack of ventilation, in basements and storerooms.

If you do not fix the cause, the salts come back. Cleaning only the surface helps for a short time.

How to treat it

Find and deal with the source of the water. If it is rising damp or a leak that is hard to find, ask a professional to assess it.
Wait for the wall to dry. While it is damp, the salts keep coming out.
Protect yourself with gloves, a mask and goggles, because the dust irritates the nose, throat and eyes.
Brush the salts off dry, with a stiff-bristled brush, without wetting the wall. Wetting can make the salts soak back into the wall.
Collect the dust with a dustpan and brush, or with a suitable vacuum cleaner. Throw it away.
If the plaster is soft, scrape it back to a firm area and repair it.
Apply a suitable primer and then the paint.

Tip: If the crystals come back after a few days, the wall still has water in it. Repeat the brushing and wait longer before painting.

What not to do

Do not paint directly over the salts. The paint will not bond and will fall off.
Do not wash the wall with lots of water, unless a professional tells you to.
Do not apply plaster or filler over a wall that is still damp.
Do not assume the problem is only cosmetic. Salt deposits damage the plaster and the paint over time.

When to ask for help

If the salt deposits cover a large area, keep coming back or come with plaster falling off, it is better to ask for an assessment. In many cases, the solution involves work on the plaster, the drainage or the base of the wall itself. A professional can tell you which makes sense in your home.

Only once the wall is dry and stable will the paint last.