Most paint failures on exterior walls have nothing to do with the paint itself. They come down to what was underneath it when it was applied: algae, dirt, chalky old paint residue or a greasy film that stopped the new coat bonding properly. If you are about to have your render, pebbledash or brickwork painted, cleaning it properly first is not optional prep, it is the single biggest factor in how long the job lasts.
Here is what a proper pre-paint clean involves, why most decorators insist on it, and what happens when it gets skipped.
Why a soft wash before painting matters more than the paint
Paint needs a clean, sound, dry surface to key into. Algae, lichen, cobwebs, pollution grime and loose flaking paint all sit between the new coat and the wall, and no amount of extra coats fixes that. Within a year or two, paint applied over a dirty surface tends to peel, blister or show the old dirt patterns bleeding through, especially on north-facing elevations that stay damp longer.
A soft wash uses a low-pressure rinse combined with a biocide-based cleaning solution to lift dirt and kill algae and spores at the surface, without blasting water into the render or driving moisture into the wall build-up. This is why almost every decorator working on rendered or pebbledash properties will either ask you to have this done first or factor it into their own prep, usually at a higher price than a specialist cleaning firm would charge for the same job.
What a pressure washer alone gets wrong
It is tempting to hire a pressure washer and blast the walls yourself the weekend before the painters arrive. The problem is that high pressure on render, pebbledash and older brick pointing can drive water behind the surface, crack pebbledash keying, and strip sound old paint unevenly, leaving a patchy surface that then needs more prep, not less.
It also does nothing about the biological growth causing the problem in the first place. Blast algae off the surface and it looks clean for a few weeks, but the spores are still in the wall texture and can start showing through fresh paint within months as it starts regrowing underneath.
Timing it right before the painters arrive
Walls need to be properly dry before paint goes on, so a soft wash should happen at least a few days before painting starts, longer in damp weather. Booking the clean and the paint job back to back without a drying gap is one of the most common causes of adhesion problems we see traced back after the fact.
It is also worth checking the wall for any loose render, cracks or areas of blown pointing while it is clean and easy to inspect properly. Painting over damage just hides it temporarily; a decorator or builder can spot and flag repairs far more easily on a freshly cleaned surface than a dirty one.
What it typically costs and who should do it
A full soft wash of an average semi's rendered elevations typically runs £250-£500 depending on size, access and how much biological growth is present, less than the cost of redoing a peeling paint job a couple of years early. It is worth booking as a distinct job rather than assuming your decorator's own wash-down covers it, since specialist equipment and biocide products get a more thorough result.
This is one job we would steer most homeowners towards a professional for rather than DIY, purely because getting the pressure and chemical dwell time wrong on render can cause damage that costs far more to put right than the clean itself.
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