You’ve done all the right prep steps, so the fact that this one small patch keeps “rejecting” paint and turning powdery means there’s probably something wrong beneath the surface, not the paint itself. from your description — powder reappearing, paint not bonding, plaster not holding — the most probable causes are: persistent moisture coming through the wall (from your neighbour’s side, plumbing, or condensation).
• even slight damp can migrate through plaster and push paint off as a chalky powder.
• this would explain why it takes days to appear again after painting. efflorescence (salts coming through the wall).
• if moisture is moving through masonry, it can carry salts that crystallize on the surface, forming a white powder that looks like chalk dust.
• painting over efflorescence never works long-term — it’ll keep reappearing. contaminated or unstable substrate.
• if at some point that patch was treated with an incompatible product (oil, old damp-seal, adhesive, or even cooking grease that soaked in), it can stop anything from bonding properly. step 1: strip it back
• scrape back all loose material until you reach sound plaster or masonry.
• if it’s plastered but soft, remove that section back to brick/block.
• let it dry completely (can take days).
step 2: neutralize efflorescence or salts (if present)
• brush the area with a stiff nylon brush and warm water to remove salts.
• let dry fully.
• avoid detergents — they can worsen it. step 3: seal the wall appropriately
depending on what you found:
• if moisture is coming through:
use a breathable damp-seal primer (e.g. zinsser watertite, dryzone damp-resistant primer) that allows some vapour movement but blocks salts.
do not use pva or standard sealer — it’ll trap moisture and make things worse.
• if the wall is dry and stable:
use a stabilising solution for chalky surfaces (like leyland trade stabilising solution). step 4: replaster (if needed)
• use a cement-based or waterproof plaster repair mortar (not regular gypsum) if the wall tends to get damp.
• let it cure fully.
step 5: repaint
• apply a quality breathable emulsion (avoid vinyl silk/glossy finishes).
• two thin coats after full drying.