Repainting Before Diwali? Test the Wall for Damp First

The rains ease off, Diwali is on the calendar, and the walls look tired. So the painter gets booked. Here is the problem: in Bangalore, an exterior wall can look dry and still be holding water from the last three months. Paint it now and you are paying for a finish that starts to bubble by next monsoon. A ten-minute test before you buy a single litre of paint saves the whole job.

Why a dry-looking wall can still be wet

Masonry and plaster soak up water slowly and release it slowly. A wall that took driving rain for weeks can keep moving moisture outward long after the surface feels dry to the hand. That moisture has to go somewhere. If there is fresh paint on top, it pushes against the film and lifts it, which is the blistering and flaking you see a few months later. It is the same mechanism behind the white powder we covered in our note on efflorescence.

Paint does not stop water. It only hides the fact that it is there, for a while.

The 48-hour plastic-sheet check

You need a piece of clear polythene about 30 cm square and some strong tape. No tools, no meter.

1. Pick your worst spots. Bottom of the wall near the ground, below windows, under the parapet, any wall that shows staining or was damp inside during the rains. Test two or three places, not one.
2. Tape the sheet flat on all four edges. Seal it fully so no air gets in or out. Do this on a dry, sunny day.
3. Leave it 48 hours. Do not touch it. Avoid days when rain is forecast.
4. Read the result. Droplets or mist on the inside of the sheet, or a darker patch on the wall underneath, means moisture is still coming out. A clear sheet and unchanged wall colour means that spot is dry.

The right order of work

If the test is clean, you can move on. If it is not, the sequence matters more than the brand of paint.

• First, stop the water. Trace the source and fix it: seal cracks, redo failed joints, repair leaking pipes or drains. Our post on hairline and structural cracks covers how to tell which cracks are which.
• Second, let the wall dry out. Give it dry weather and repeat the plastic-sheet check. There is no honest fixed number of days, because thickness, exposure and the size of the leak all change it.
• Third, repair and prime. Scrape off loose paint and any white salt, fill cracks with a flexible filler, then apply the primer.
• Last, paint. A good exterior coat is the finishing layer, not the waterproofing.

When waterproofing should come before paint

If the same wall has peeled in the last two years, if a terrace or parapet sits above it, or if damp shows on the inside face, the wall needs a proper waterproofing treatment first. That is a job for the source, not the surface. We look at it in an inspection, tell you plainly whether the wall needs treatment or just a repaint, and will not sell waterproofing where a paint job will do.

If you want the painting done before the festival, sort the moisture check this week. That leaves time for a fix, a dry spell and the paint.

Frequently asked questions