Preparing Outdoor Concrete Before Painting: What Happens and Why
Preparation is the part of a concrete painting job that decides whether the colour stays put, and it usually takes more effort than the painting itself. A properly prepared slab is clean, sound, dry enough for the product, free of loose coatings and repaired so the new film has something solid to hold onto. This guide explains each stage so you can recognise a careful job; it is not a do-it-yourself manual.
Send us a photo and the m²Why does preparation matter more than the paint?
Because a coating bonds to whatever it touches first. If that is a layer of dust, algae, grease or flaking old paint, the new colour is stuck to something that is itself coming loose, and it will fail with it.
The best floor coating on the market will peel from a dirty slab, while a modest one on a well-prepared surface can perform exactly as its data sheet describes. That is why a careful contractor spends most of the job on what happens before the roller comes out.
What does proper cleaning involve?
Proper cleaning removes everything that is not sound concrete: soil, dust, algae and moss, loose particles, salts that have bloomed to the surface and as much staining as will come out. On outdoor surfaces this normally means pressure washing the whole area, including the joints and edges.
Oil and grease are the hardest part. Fresh drips often lift; old, deep stains may only partly come out and can affect how the coating bonds above them. A contractor should tell you honestly what will remain, and our sister site on pressure washing covers that subject in more detail.
Which defects need repairing before painting?
Anything that would show through the colour or let it fail needs repairing first. On outdoor concrete that usually means:
- Cracks: fine and medium cracks are opened up where needed and filled so they do not telegraph through.
- Chipped areas and small potholes: rebuilt so the surface is continuous.
- Open joints: refilled, especially in block paving and between slabs.
- Broken edges and kerbs: repaired so the cut-in line has something solid to follow.
Cracks that keep widening, sunken areas and rocking slabs are not cosmetic: they point to movement beneath, and painting over them is pointless until the cause is dealt with. For more on that, see our page on concrete painting and when it is the wrong answer.
What happens to old paint and sealer?
Old coatings are tested rather than assumed. If they are firmly bonded and the new product is compatible with them according to its data sheet, they can stay and be cleaned; if they are flaking, powdery or lifting, they have to be removed.
Imprinted concrete is often found under layers of old clear sealer that have turned cloudy or white in patches. Those areas need particular attention, because a new colour coat over a failing sealer will peel with it, which is the most common story behind the cases in our guide on why paint peels.
Why does moisture in the slab matter?
A film-forming coating needs a surface that is dry enough, within the limits stated on its data sheet. Water trapped in or rising through the concrete can push the film off from below, causing blisters or peeling.
After washing, the surface has to be allowed to dry before painting, and shaded or low-lying areas take longer. Persistent damp from below, for example at the foot of a slope or next to a planter, is a warning sign that needs assessing before any coating goes on.
Why are edges and cutting in part of preparation?
The edges are where a painted floor is judged. Lines along walls, posts, steps, drain covers and planters are painted by brush before the open area is rolled, and masking is used where a crisp line is needed against another surface.
On paths and patios with many borders, this brushwork can take as long as the rolling. It is one of the reasons why the shape of an area affects the time a job takes, as explained on our patio and terrace painting page.
What happens to the wash water and paint residues?
Paint residues and solvents must not end up in the drains. In Málaga city, the municipal sewer regulation prohibits discharging into the sanitation system the substances in its Annex 1, which include water-immiscible organic solvents, paints and pigments that cannot be removed by normal treatment, and solids that could block the network (Reglamento del Servicio de Saneamiento).
For Marbella and other municipalities, the specific discharge rules are to be verified, and owners should check with their ayuntamiento. On block-paved areas, see also our page on paver painting for how joints are handled during washing.
Do you need permission before the work starts?
It depends on the town and the zone. In Málaga city, the planning department's list places repairing or coating exterior slabs in gardens, pools or common areas among minor works, generally processed by declaración responsable, though protected areas may require a licence (Gerencia Municipal de Urbanismo).
For Marbella, Estepona, Mijas, Fuengirola, Benalmádena and Torremolinos the requirement is to be verified: depending on the municipality and the zone, a declaración responsable or a licence may be needed, and the owner should confirm it with the ayuntamiento before work begins.
Frequently asked questions
Is pressure washing enough preparation on its own?
Not usually. Washing removes dirt and loose material, but cracks, chipped areas, open joints and loose old coatings still need dealing with, and the surface must dry to within the product's limits. Washing is the first step of preparation rather than the whole of it.
Can paint go on straight after washing?
No. The concrete needs to dry to the moisture level allowed by the product's data sheet, which depends on the weather, the shade and the type of surface. Painting a damp slab is one of the commonest causes of blistering and peeling.
Do all cracks need filling before painting?
Cracks that would show through the colour or let water in are filled as part of preparation. Hairline marks on the surface may disappear under two coats. Cracks that keep moving, or that come with sunken areas, signal a structural problem that painting cannot solve.
Why does old sealer cause problems?
Old sealer that has lost adhesion, turned white or begun to flake gives the new coating a weak base. If new paint goes over it, both layers can come away together. Sound, well-bonded sealer can sometimes stay if the new product is compatible, which the data sheet confirms.
Does a primer replace cleaning?
No. A primer helps the coating bond to a clean, sound surface; it cannot make up for dirt, algae or loose material. Where a system requires a primer, it is applied after cleaning and repairs, never instead of them. Skipping cleaning because a primer is planned is a false economy.
How do you deal with algae and moss?
Algae and moss are removed during pressure washing, with extra attention to joints, shaded corners and areas near planters. Any green film left behind would weaken the bond of the coating. If an area stays persistently damp, that is noted as a risk before painting.
Can rust stains be painted over?
Some rust marks fade with cleaning and can then be covered, while others are deep and may show through or affect the coating. A close-up photo helps judge them. Our sister site on cleaning covers rust and moss removal in more detail.
Why is masking tape used on some jobs?
Masking protects adjacent surfaces, such as walls, door frames, pool coping or tiles, and gives a crisp line where the painted floor meets them. It is used alongside brush cut-in rather than replacing it, and removed once the coating allows.
Do I need a permit to paint my patio?
It depends on the municipality and the zone. In Málaga city, coating exterior slabs in gardens, pools or common areas is listed as a minor work usually handled by declaración responsable, while protected areas may need a licence. Elsewhere the rule is to be verified with the ayuntamiento before work begins.
Can I prepare the surface myself to save money?
We do not publish step-by-step preparation instructions, because preparation is where most failures start and the standard required depends on the product. If you want to reduce the scope, discuss it with us first; moving furniture and pots out of the way beforehand is always helpful.