Damp and problem walls need the cause fixed first, then a primer that bonds where standard primers cannot grip: Bond & Key for friable, chalky surfaces, Damp Proof for interior damp walls, and an exterior waterproofer like Shield to keep moisture out. Primer alone cannot fix structural moisture. This guide covers how to diagnose the damp, prepare the surface, and choose the right primer-and-topcoat system so the paint job lasts.
Diagnose the damp before you prime
Fix the source of the damp first, because no primer or paint holds on a wall that moisture keeps entering. Damp comes from four sources, each with its own fix:
- Rising damp — ground moisture wicking up the wall; needs a damp-proof course or membrane below the affected area.
- Lateral or penetrating damp — driving rain or cracks; seal the exterior with Shield, or Megaflex on parapets and wet areas.
- Condensation — interior humidity; improve ventilation, because paint will not fix it.
Identify which one you have before spending on primer, or the coating fails again. The full diagnosis is in the guide to identifying and fixing damp walls.
Prepare the problem surface
Strip the wall back to a sound, dry, clean surface before priming. The steps are: remove all loose, flaking or chalking paint (if a chalky layer rubs off on your hand, it must come off); treat mould with a fungicidal wash and let it dry; brush off efflorescence (white salt deposits) with a stiff dry brush without wetting it back into the wall; repair cracks with a suitable filler; and let the wall dry fully. Moisture content should be within the primer's spec, typically below about 16% — check a doubtful wall with a moisture meter.
How do you know a wall is dry enough to paint?
A wall is dry enough to paint when its moisture content is within the coating's spec, usually below about 16%, and it shows no fresh damp patches. The reliable check is a moisture meter across several points, especially low on the wall where rising damp shows. A quick field test is the plastic-sheet method: tape a square of clear plastic tightly to the wall overnight; condensation or a darkened patch underneath means the wall is still too wet to coat. Do not paint on the strength of a surface that merely looks dry — damp hides below.
Apply the right primer
Use a bonding primer on friable and chalky surfaces, and an alkali-resistant plaster primer where the wall is cement-rich. Bond & Key is made to bond to friable, chalky and difficult surfaces that standard primers slide off; it cures into the surface and gives the topcoat a stable base. On cement-rich or alkaline walls, prime with Plaster Primer first to neutralise alkalinity. For a chalky, dusting surface, Clear Bonding Liquid binds and seals it. Apply per the label and do not over-thin.
Efflorescence and salt damp
Efflorescence is the white, powdery salt that migrates to a wall's surface as damp evaporates, and it must be removed dry before priming, not painted over. Brush it off with a stiff dry brush; do not wash it, which drives the salts back into the wall to reappear later. If efflorescence keeps returning, the wall is still drawing moisture and the source has not been fixed — a coating over active salt damp lifts as the salts push through. Deal with the moisture and let the wall dry before you prime and paint.
Treating interior damp walls
Interior damp walls take Damp Proof, a coating made to treat damp on inside walls once the source is under control. It suits rising-damp and condensation-prone interior faces that a normal wall paint would peel off. On the wet side of the problem — the exterior wall — the fix is to keep water out with a waterproofer, while Damp Proof handles the interior face.
Topcoat with the right product
Match the topcoat to the room: washable finishes for damp-prone interiors, and a waterproof exterior coat where lateral damp is the problem. Bathroom and kitchen walls take a washable finish like UltraLux or High Sheen Enamel that resists moisture; wet areas behind tiles take Megaflex. Exterior walls facing driving rain take Shield, whose flexible, waterproof film stops moisture re-entering the wall.
What a primer cannot do
A primer cannot fix active rising damp, ongoing penetrating damp, or unresolved condensation. Primer and paint go onto a stable, dry surface. If moisture keeps entering the wall from elsewhere, the paint film will fail no matter which primer you used. This is why the diagnosis and the source fix come first, and the primer-and-topcoat system second. Treat the two as one job: stop the water, then coat the wall.
Where to buy
Shop Bond & Key, Damp Proof and the full Primers range with national delivery, or visit our paint shops in Table View, Cape Town and Edenvale, Johannesburg. For advice on a damp problem, call +27 84 985 6141. See the full paint primer guide.
