A Story from the Field: Freshly Painted, and the Black Spots Were Back in Three Weeks

Khun Somsak is a 65-year-old retired civil servant who lives with his wife in a two-storey detached house off Wutthakat Road in Chom Thong, on the Thonburi side of Bangkok. During the 2026 floods, water from a nearby canal came into the ground floor and stood about forearm-deep for five days. The ground floor has two kinds of walls: the original brick walls finished with cement plaster around the outside, and lightweight gypsum partitions that were added later to separate a study from a storeroom.

As soon as the water drained, Khun Somsak hired a local painter to scrub the stains and repaint the whole ground floor within a week. He wanted the house to look like home again before the grandchildren came to visit. Just three weeks later, the paint on the gypsum partition began to bubble, and a line of black mold appeared just above the skirting. Parts of the plastered walls developed white salt stains and small black specks of their own. His wife, who has allergies, woke up congested every morning, and the study smelled so musty that nobody wanted to use it.

When the WHD team surveyed the house, we pressed on the gypsum partition and it gave way under a thumb. Once a section was opened, the paper facing on the back of the board and the timber studs behind it were covered in black mold. The plastered walls, by contrast, were structurally sound but still damp inside. "I paid for a paint job for nothing," Khun Somsak said, "because nobody told me the two kinds of wall need completely different treatment."

Khun Somsak's painter was not dishonest or careless. He did exactly what he was asked to do, quickly and neatly. The problem was the order of work: the walls were painted before anyone had checked what was happening inside them, and before they had been dried and disinfected.

Many households in Chom Thong, Wutthakat, Ekkachai, Bang Khun Thian and Bang Mot are facing exactly the same situation. This article explains which walls can be repaired, which should be removed, and the correct order of work before you pick up a paint roller.

Why Black Mold on Walls After a Flood Is More Than a Cosmetic Problem

• Health. Mold spores in the air can trigger allergies, asthma, persistent coughs and eye or skin irritation, particularly in older people, children and anyone with a weakened immune system.
• Contamination from floodwater. Water that soaked into walls often carried germs from drains and canals, not just mold.
• Spreading damage. Mold inside gypsum partitions feeds on the paper facing and spreads to timber studs, built-in cabinets and door frames.
• Paying twice. Paint applied over walls that are still damp or still moldy will blister, the mold will return, and you will pay the painter again.
• Persistent musty odour. Odours from mold inside walls soak into curtains, mattresses and furniture until the whole house smells.

Many houses in Chom Thong have been extended several times, so a single floor often contains both original plastered masonry and lightweight partitions added later. Treating every wall the same way is one of the main reasons the problem keeps coming back.

Know Your Walls: Cement Plaster vs Gypsum Board

Brick or concrete walls finished with cement plaster

These are strong materials that do not disintegrate when wet, but they are porous. They soak up water and hold moisture for a long time. After a flood they may feel dry on the surface while remaining damp inside for weeks. Mold tends to grow on the paint film or the plaster surface while it is still damp, and you may see white, powdery salt deposits as moisture works its way out.

General approach: in most cases these walls can be repaired without demolition, provided they are cleaned, genuinely dried out and disinfected in the right order.

Gypsum board (drywall) partitions

Gypsum board is a core of gypsum sandwiched between two layers of paper. After prolonged soaking, the board absorbs water, softens and swells. The paper facing is an excellent food source for mold, and the inside of a partition is a closed, dark cavity where growth can continue unnoticed.

General approach: if gypsum board sat in floodwater for several days, feels soft or crumbles when pressed, has swollen, or has mold on the back, the damaged section should usually be cut out. The cut is typically made a reasonable distance above the visible water line, after which the studs and any insulation inside are inspected.

Assessment Checklist: Repair or Remove?

This table is general guidance. Real decisions depend on what is found on site: how high the water rose, how long it stayed and the condition of the structure. If you have any doubt about structural safety, consult a builder or engineer.

Why the Usual Fixes Fail

• Scrubbing and repainting straight away. The wall is still wet inside, so paint blisters and mold returns.
• Wiping with bleach alone. The colour of the mold may fade until it looks gone, but bleach penetrates porous materials poorly and adds yet more water to the wall.
• Keeping swollen gypsum because it seems wasteful to remove it. Mold on the back of the board keeps growing and the smell never leaves.
• Pointing a fan at the wall. The surface dries while the inside stays damp, and spores are blown around the house.
• Air fresheners. They mask the smell for a while but do nothing about its source.

Signs That Mold May Be Hiding Inside a Wall

The most expensive mold is the mold you cannot see. Before deciding a wall is fine, look and feel for these warning signs:

• A musty smell that is noticeably stronger when you put your nose close to one section of wall, or when you open a cabinet fixed to it.
• Paint that is bubbling, peeling or developing a faint grey or brown shadow, even without obvious black spots.
• Skirting boards that have swollen, pulled away from the wall or developed dark marks at the joints.
• A gypsum partition that sounds dull or feels slightly spongy when tapped or pressed near the floor.
• Allergy symptoms in the household that are worse in one particular room.

If several of these signs appear together, it is worth opening a small inspection hole in a gypsum partition, or removing a section of skirting, before committing to repainting. A small hole now is much cheaper than a full rebuild later.

The Right Order: From Moldy Wall to Ready-to-Paint

1. Assess. Identify which walls are plaster and which are gypsum. Check for softness, swelling and odour.
2. Protect yourself first. Wear an N95 respirator, gloves and goggles, especially when scraping or removing moldy material, and seal off the work area so dust does not spread to other rooms.
3. Cut out damaged gypsum. Cut a reasonable distance above the water line, remove wet insulation, inspect the studs, and bag debris securely before carrying it out.
4. Scrape failed paint and plaster. On masonry walls, remove blistered paint and crumbling plaster and clean off residues.
5. Dry properly. Ventilate, use fans and dehumidifiers, and give the wall time to dry right through. A moisture meter helps confirm progress.
6. Disinfect and ozone-treat. Treat surfaces, the open wall cavities and the air.
7. Repair and repaint. Fit new board, patch plaster, and use primers and finishes suitable for humid areas, only once the wall is fully dry.

The key point is to disinfect while the wall is still open, before new board goes on or fresh paint goes over the top, so that mold is not sealed inside.

How Long Does a Flooded Wall Take to Dry?

This is the question every homeowner asks, and the honest answer is: longer than it looks. A plastered brick wall that stood in water for several days can stay damp inside for weeks, depending on its thickness, the paint on it, how well the room is ventilated and the weather. Bangkok's high humidity during and after the rainy season slows drying considerably.

Rather than relying on a fixed number of days, judge readiness by evidence. Surfaces should feel dry and cool rather than cold and clammy; any white salt bloom should have stopped growing; the musty smell close to the wall should have faded; and, ideally, a moisture meter should show readings that have levelled off at a low value across the wall, not just at the top. Running a dehumidifier in a closed room, with furniture pulled away from the walls, speeds things up far more than fans alone.

It is tempting to rush. Grandchildren are visiting, a tenant is waiting, or the shop needs to reopen. But painting a wall that is not ready almost always means doing the job twice. A few extra weeks of drying is cheaper than a second paint job and a second round of mold treatment.

Places That Are Often Missed When Repairing Flooded Walls

• Behind built-in cabinets fixed to the wall. These are dead-air spaces where mold grows easily. Move or remove them to inspect.
• Timber skirting and door frames. They wick water from the floor and wall, and mold can grow behind the skirting where it cannot be seen.
• Low-level sockets and switches. They must be checked by an electrician before use, and they are openings through which moisture reaches the wall cavity.
• Insulation inside partitions. Wet insulation holds water for a very long time and should be removed and replaced.
• Ground-floor ceilings. Even if water never reached them, rising humidity can cause mold on gypsum ceiling boards.

Working with Your Builder: A Simple Handover Plan

Wall recovery usually involves at least two parties: the builder or painter who removes and rebuilds, and the disinfection team. Problems arise when nobody owns the order of work. A simple plan avoids that:

1. Joint survey. Ideally the builder and WHD see the house in the same week, so the cut line for gypsum, the walls to be scraped and the disinfection scope are agreed together.
2. Demolition day. The builder cuts out damaged board, removes wet insulation and scrapes failed plaster, with the area sealed off and debris bagged.
3. Drying period. The open walls are left to dry with dehumidifiers and ventilation. This is not wasted time; it is when the wall becomes repairable.
4. Disinfection and ozone day. WHD fogs the open cavities, studs, plaster surfaces and floors, then ozone-treats the sealed rooms and manages ventilation.
5. Rebuild and repaint. Once the walls are confirmed dry and the treatment is complete, the builder closes the walls and repaints.

Writing these steps on a single page and sharing it with everyone involved saves arguments later about who should have done what, and when.

Choosing Materials for the Rebuild

If you are replacing gypsum partitions on a ground floor that could flood again, it is worth asking your builder about moisture-resistant board, leaving a small gap between the bottom of the board and the floor, and using skirting that can be removed for inspection. On plastered walls, primers and finishes designed for humid areas help, but only when applied over a dry, clean, treated surface. No coating can make up for painting over a wet wall.

Preventing Mold from Returning After Repainting

Once the walls are repaired, keep windows open during dry daytime hours, leave a little space between furniture and walls, run a dehumidifier in rooms that feel stuffy, and keep an eye on the spots where mold grew before. If a musty smell or black specks return, look for the moisture source first, such as a leaking pipe or water rising through the floor, before the mold has a chance to spread again.

WHD's Service: Chemgene HLD4H Fogging Plus Ozone, Before the Walls Are Closed

World Health Disinfection (WHD) steps in at the most important moment: after damaged material has been removed and before walls are closed up or repainted.

1. Site survey. We check the water level, wall types, where mold is growing and what should be removed.
2. Removal and cleaning advice. We coordinate with your builder on how high to cut gypsum and what to clean before the disinfection day.
3. ULV disinfection fogging with CHEMGENE HLD4H. The fine mist reaches plaster surfaces, open wall cavities, studs, floors and corners.
4. Ozone shock treatment. The room is sealed; electrical appliances, artworks and rubber items are covered or removed first; and the ozone dose is controlled for the room size. This reduces airborne spores and breaks down musty odours embedded in materials.
5. Ventilation. Doors and windows are opened until the ozone has broken down, before residents and builders return.
6. Re-check. We check odours and risk points and advise on the right moment to close the walls and repaint.

Ozone safety: no people, pets or plants may be inside during treatment, and the area must always be properly ventilated before anyone goes back in.

10 Reasons to Disinfect Before You Repair the Walls

1. Mold is not sealed in behind new gypsum board or under new paint.
2. ULV fogging reaches open wall cavities and studs.
3. CHEMGENE HLD4H means no mixing of chemicals on site.
4. Ozone treatment reduces spores stirred up during demolition.
5. Musty odours embedded in plaster and nearby furniture are broken down.
6. Less chance of blistering paint and returning mold, so you do not pay for painting twice.
7. Older family members and anyone with allergies are better protected.
8. Electronics are covered or moved before treatment and the ozone dose is controlled.
9. We can coordinate with your builder so each step happens at the right time.
10. We are easy to reach by phone and LINE; read more about residential ozone disinfection.

Customer Voice

"The first time, I rushed the painting and wasted my money. This time the WHD team advised my builder to cut out the damaged gypsum first, then came in to fog and ozone-treat while the walls were still open. Once the walls were closed and repainted, the musty smell was gone, and my wife breathes much more easily."

— Khun Somsak, detached house off Wutthakat Road, Chom Thong (a story from the field)

Frequently Asked Questions

Q: My gypsum wall was flooded but still looks fine. Does it have to come out?
A: It depends on how long it was soaked, whether the water was clean or contaminated, and whether there is mold on the back. If the board is soft, swollen or smells strongly musty, that section usually needs to be cut out. We can assess it on site.

Q: How long should a plastered wall dry before painting?
A: It depends on wall thickness, how long it was submerged and how well the room is ventilated. Wait until it is genuinely dry inside, ideally confirmed with a moisture meter, rather than painting because the surface looks dry.

Q: Can I wipe the mold off with bleach?
A: It may lighten the surface colour, but it penetrates porous materials poorly and adds moisture to the wall. Never mix it with other products. Focus on drying, removing damaged material and disinfecting the whole space.

Q: Should I call WHD before or after the builder opens up the walls?
A: Ideally after damaged sections have been removed and before new board goes on, so we can treat the cavities and studs. We can also survey beforehand to plan the work with your builder.

Q: Will ozone damage the wiring inside the walls?
A: We cover or ask you to remove sensitive appliances and control the ozone dose for the space. Any wiring that was under water should be checked by an electrician before use in any case.

Q: Is black mold on walls always the dangerous kind?
A: The colour alone does not tell you which species it is, and many molds look dark. Rather than worrying about identification, treat all post-flood mold as something to remove and prevent, using protective equipment while you work.

Q: Do you serve Chom Thong and Bang Khun Thian?
A: Call 065-556-6294 or LINE @whd268 to check availability and arrange a survey.

Before You Close the Walls or Repaint, Let WHD Disinfect First

The WHD team is ready to survey homes in Chom Thong, Wutthakat, Ekkachai and nearby areas, and to work alongside your builder so every step happens in the right order.

Book Ozone Treatment – Click Here

See Our Disinfection Service – Click Here

Call 065-556-6294 · LINE @whd268 · Email worldhealthdisinfection@gmail.com

Related Services and Reading

• Residential ozone disinfection
• Ozone treatment for odours and chemical residue
• Master Ozone Generator
• All articles
• External sources: WHO fact sheet: Floods and Department of Health, Thailand