Must Old Coatings Come Off Before a New Render System?

What the new system needs underneath

Start from the data sheet of the system you are considering, because it states the background it needs. K Rend's silicone finish asks for backgrounds that are clean, level, dry and free from efflorescence. Its K Mono monocouche asks for substrates that are clean, sound and free from material that may reduce adhesion. Neither describes an old flaking paint film or a patchwork of hollow render.

That gives a simple first test. If the existing coating could be described as clean, sound and well bonded, a new system may be able to go over it, usually after a base coat. If it is loose, hollow, powdery or flaking, it has to come off, because new render bonds to the surface beneath it and will fail along with it.

Dashed walls need particular care. If pebbledash covers only some elevations, test each one separately; a front wall of dash and a smooth painted rear may share a colour but have very different histories and bond. A sound dash can sometimes be levelled with a base coat and refinished, while a hollow dash on another wall of the same house is stripped. One house can need both answers.

Pebbledash removal ↗

1. 1

   Read the new system's data sheet for the background it needs.

2. 2

   Tap and scratch-test each coating, wall by wall.

3. 3

   Trial-remove a small area where removal is proposed.

4. 4

   Record what stays and what comes off before covering it.

Coating by coating

Masonry paint over render is the most common case. Well-bonded paint can usually be cleaned and recoated, or covered by a system whose maker accepts a painted background. Paint that flakes, blisters or leaves powder on your hand has to be removed or stabilised first, and a monocouche will not bond to it at all.

Thick textured wall coatings, sometimes sprayed on decades ago, can trap moisture and hide hollow render beneath. They are tested in the same way, by tapping and by cutting a small inspection hole, and often come off with the render behind them.

Sound cement render can often stay as the background for a new finish, provided it is clean, sound and true enough. The silicone page explains why a thin finish follows any unevenness in the surface below. Lime render and limewash on an old wall are usually kept and repaired with lime, rather than being stripped and covered with a cement-based system.

When removal does more harm than good

On old soft brick and stone, removing a hard coating can do real damage. SPAB advises taking off a small trial area first, because removal may harm the fabric beneath. Where cement render is still firmly stuck, SPAB suggests it may be better left to age, or removed only at the base of the wall and in hollow-sounding patches. Levering off large sheets can tear the faces off soft materials; hammer and chisel work is slower but kinder.

Removal also brings what it finds. SPAB notes that a wall that has worn cement render for years may hold salts, which a new lime render can draw out, so some local re-rendering a couple of years later can be expected. The pebbledash removal guide covers what typically appears behind dash, from good brick to old repairs and infilled openings.

A worked example

For example, imagine a hypothetical post-war house with three different coatings. The front is pebbledash, the side gable is cement render painted many times, and the rear is a small extension in painted blockwork. The owner wants one new silicone finish throughout.

Tapping finds the front dash hollow over most of its area, so it is stripped. The side render taps sound, but the paint flakes when scratched; the loose paint is removed, the render is washed and repaired, and the wall is given a meshed base coat so the thin silicone finish has a clean, level background as its data sheet requires. The extension blockwork is sound, but its paint film would stop a new base coat bonding, so the paint is removed mechanically before the base goes on.

Three walls, three preparations, one finish. The owner's record notes which walls were stripped to brick, which kept their old render under the new base, and what was found behind the dash, so that a future repair on any wall starts from the right assumption.

Record what stays

If an old coating stays under the new work, write down what it is, where it is and how it was tested. Once the new finish is on, nobody can see the difference between a stripped wall and a coated one, but the next person to repair it will need to know.

Take photographs of trial areas and stripped walls, with dates, and keep the product names of whatever is applied. The Doncaster homepage links the coating, repair and replacement routes, so the record can point to which one each wall received.