A garden wall takes rain, sun, irrigation and soil splashes, and often one face never dries. Paint fails sooner than on a house facade. This guide covers what to do before painting, which paint to use, how many coats, which days to avoid and what is known about permits.
Updated 4 November 2026. General information, not a substitute for a technician’s assessment or legal advice.
The essentials
- Water first, paint second. White salts and algae only appear where there is water. If it keeps getting in, new paint will fail again.
- The top is the weakest point. A conservation article on garden walls says the head of the wall, with coping and a drip, is what protects it most from rain.
- Use exterior masonry paint and follow the data sheet: in the sheets we read, above 5 ºC, air humidity below 75% to 80%, no rain and no strong sun.
- Two coats is the minimum in all three sheets we read.
- Building or altering a wall is not the same as painting it. Rules differ by municipality: ask the câmara (town council).
1. Work out why the wall is like this
| Sign | What it usually means |
|---|---|
| White powder or crust | Salts carried by water. See salt on walls |
| Green or black, on the shaded face | Algae and fungi, where the wall stays damp |
| Paint lifting at the base | Water from the ground, or soil banked against the wall |
| Paint lifting at the top | Water entering from above: no cap, or a broken one |
Sherwin-Williams (a paint maker) explains that crystals keep forming while salt and moisture are both present, and says to stop the water source before painting. If efflorescence keeps coming back, delay painting. For banked soil, see soil against the wall and drainage.
2. Clean: loose paint, salts and algae
- Loose paint. Scrape and brush. The Robbialac sheets say to remove old paint that is breaking up and to remove limewash completely. If the old paint is of unknown age, do not dry-sand it: see old paint and lead.
- Salts. The Barbot sheet says, for salt deposits, to remove loose paint, brush the surface salts off thoroughly, and only then apply the brand’s sealer. Sherwin-Williams accepts a wire brush or a low-pressure wash with a good rinse. Acids: only on a professional’s advice.
- Algae and fungi. The Aquarepel 3Tech and Siloxan NG sheets give the same sequence: scrape, wash with water and bleach, rinse, let dry, then apply the maker’s disinfectant. For severe cases they allow a high-pressure water jet. Wear gloves and goggles, work outdoors, and never mix bleach with other cleaners.
- Let it dry. We found no official drying time. Paint only when the wall feels dry and shows no dark water patches.
3. Repair cracks, joints and the top
- Cracks. Barbot says that for cracks up to 2 mm you open the crack slightly with the edge of a spatula and fill it with exterior filler. Above 2 mm, open and clean before treating; cracks that move need an elastic filler. See cracks and plaster before painting.
- Joints. The conservation article (Jonathan Taylor, 2002) prefers lime mortars and discourages adding cement, which makes them less permeable. It speaks of old stone walls; for a recent block wall, ask the block maker or a mason.
- Top. The same article says to replace missing copings, with a drip, so water runs away from the face. We found no official Portuguese dimension.
Repair the top before painting. Painting a wall that rain still enters from above is wasted paint.
4. Choose the paint
| Product | Intended for | From the sheet |
|---|---|---|
| Aquarepel 3Tech (Robbialac, undated) | Outside walls, new work and repainting | Acrylic, matt; water W3 (low), vapour V2 (medium) |
| Siloxan NG (Robbialac, CC 462 20.1) | Limestone, cement, stucco, render; older buildings | Siloxane; vapour V1 (high); full water repellence only after about 28 days |
| 3300 Barbolite (Barbot, 02/12/2024) | Exterior render with alkalinity and efflorescence | Solvent-based acrylic; V2 and W3; over powdery render, with the brand’s primer |
In plain terms: on smooth, stable, dry render, an acrylic facade paint will do. On an old stone or lime wall, prefer paints that let vapour out (siloxane, or lime paint), because the conservation article warns that waterproof coatings can trap water and worsen salt damage. More on classes and finishes in exterior and facade paint.
For quantities use the calculator. The sheets give 10 to 12 m² per litre per coat (Aquarepel), 8 to 10 (Siloxan) and 6 to 8 (Barbolite). A rough surface uses more.
5. Coats and intervals
| Sheet | Coats | Thinning | Between coats |
|---|---|---|---|
| Aquarepel 3Tech | at least 2 | first coat up to about 10% water | 2 to 3 hours (about 20 ºC, ventilated) |
| Siloxan NG | 2 to 3 | up to 10% | 12 hours; 8 to 10 days to dry fully |
| 3300 Barbolite | 2 | up to 5%, maker’s thinner | 8 hours |
Siloxan NG asks for a bonding primer to keep vapour permeability. Application steps are in how to paint a wall.
6. When not to paint
- Temperature: nothing below 5 ºC. Aquarepel and Siloxan stop at 35 ºC.
- Air humidity: below 75% (Aquarepel) or 80% (Siloxan, Barbolite).
- Dew: the surface must be 2 to 3 ºC above the dew point. Avoid late afternoon and mornings when the wall is wet.
- Rain: Aquarepel says not to apply in rain or if rain is forecast in the following days. For Siloxan, rain during drying can leave sticky runs, which wash off.
- Direct sun and strong wind: avoid.
- Substrate moisture: Barbolite asks for below 5%.
See also best time of year to paint.
7. Permits and the câmara
- For buildings, repainting is conservation work exempt from a licence, with notice to the câmara (RJUE, the urban development law; see painting the facade). We did not confirm that a boundary wall counts as a building in that text.
- An architects’ page says boundary walls up to 1.80 m, inside the plot and not facing the street, generally need no licence to be built, and reminds readers that municipal rules can add limits. That is professional opinion, not the text of the law.
- Historic zones, listed buildings and easements may limit colour. We read no municipality’s regulation.
Write to the câmara before changing the colour of a wall that faces the street. If the wall is shared with a neighbour, agree the work and colour in writing.
Quick plan
- Find where the water comes from and fix it.
- Clean: loose paint, salts dry, algae by the sheet’s method.
- Repair cracks, joints and the top.
- Let it dry.
- Apply the primer the sheet asks for.
- Paint on a dry day, between 5 and 35 ºC, out of direct sun.
- Give at least two coats and keep drizzle off for the next days.
What we could not confirm
- An official drying time for a washed or repaired wall.
- The current RJUE text on boundary walls and whether repainting a wall is exempt. Municipal regulations not read.
- Official Portuguese dimensions for coping.
- The date of the Aquarepel 3Tech sheet (PDF has no readable date).
- Prices for paint and labour: no dated source, so none included.
Sources (read on 4 November 2026)
- Tintas Robbialac, Aquarepel 3Tech, data sheet (CC 560 25.1, no readable date): preparation, coats, conditions, coverage.
- Tintas Robbialac, Siloxan NG, data sheet (CC 462 20.1, PDF from April 2020): substrates, primer, coats, limits, drying.
- Barbot, 3300 Tinta Barbolite, data sheet (02/12/2024, rev. 6): paint systems, salt, cracks, conditions.
- Sherwin-Williams, Efflorescence: white crystal deposits: causes, removal, prevention (manufacturer, secondary source).
- Jonathan Taylor, The Maintenance of Garden Walls, Building Conservation (from Historic Gardens, 2002): top of wall, joints, waterproof coatings.
- AC Arquitetos, Muros de vedação: precisamos mesmo de licença?: secondary source; does not cite the RJUE article.
- Diário da República, Decreto-Lei n.º 555/99 (RJUE): cited through our guide on painting the facade, not read directly today.