How Do You Prepare Exterior Surfaces for Painting? Masonry, Timber, and Metal

Summary

Exterior painting failures — early peeling, blistering, flaking, and rapid fading — are almost always the result of inadequate preparation rather than poor-quality topcoat. The primer and preparation phase dictates the adhesion and longevity of the entire paint system, yet it is frequently under-resourced relative to the topcoat application, particularly when price pressure is a factor.

The requirements differ significantly between substrate types. Masonry preparation is primarily about achieving chemical stability (neutralising alkalis, eliminating biological growth, stabilising friable surfaces) before a masonry primer or stabilising solution is applied. Timber preparation is about eliminating all trace of rot, sealing resin-exuding knots, and using the correct primer for the wood species. Metal preparation is about mechanical surface profiling, removing corrosion, and using a primer with the right inhibitor chemistry for the metal type.

All three substrate types share the requirement for thorough cleaning before any other preparation. Surface contamination — oil, grease, bird fouling, chalk, loose material — will prevent any primer or paint from achieving adhesion regardless of how well the subsequent work is done.

Key Facts

• Power washing — Use at 100–150 bar (1,450–2,175 psi) for masonry; lower pressure for aged render and soft stone to avoid damaging the surface
• Fungicide treatment — Apply proprietary masonry biocide (Ronseal Problem Wall Fungicidal Wash, Wykamol, or similar) and allow to work for 24–48 hours before washing off; repeat if growth is dense
• Sandtex stabilising solution — For chalky, dusty, or friable masonry surfaces; brush-applied; allow to dry before priming or topcoating
• Alkali neutraliser — Fresh cement render and new concrete can have pH above 12 (strongly alkaline); this attacks conventional oil-based binders; treat with proprietary alkali neutraliser (acidic wash) or allow at least 6 months for the pH to drop before painting with oil-based products
• Knotting solution — Shellac-based sealer applied to resinous knots in softwood timber before priming; prevents resin bleed through the paint system; typically 2 coats
• Timber primers — Aluminium wood primer (for hardwoods with high tannin — teak, iroko, oak, western red cedar), white/pink oil-based primer (for general softwood), acrylic primer for water-based systems
• BS 7079 surface preparation grades for steel — Sa 1 (light brush-blast), Sa 2 (thorough blast), Sa 2.5 (very thorough — most common commercial spec), Sa 3 (blast to bare metal); for hand tools: St 2 (thorough hand/power-tool cleaning), St 3 (very thorough power-tool cleaning)
• Red oxide primer — Traditional iron oxide-based primer for ferrous metals; now largely superseded by zinc phosphate primers; still used for general maintenance work
• Galvanised metal primer — Galvanised steel (zinc-coated) requires a specific primer with good adhesion to zinc; conventional red oxide primers have poor adhesion to galvanised surfaces; use a proprietary galvanised metal primer or etch primer
• Rust converter — Converts iron oxide (rust) to iron tannate; suitable for light surface rust where mechanical removal is not practical; not a substitute for thorough preparation on heavily corroded surfaces
• Lead paint pre-1960 — Houses built before approximately 1960 are likely to have lead paint layers; sanding, abrading, or stripping creates lead dust, which is a serious health hazard; COSHH assessment required; use wet methods to suppress dust; consider encapsulation rather than removal where the existing surface is still sound
• Scaffold safety — Work above 2m requires either scaffold, working platform, or tower scaffold (PASMA-trained for mobile towers); ladders should not be used for prolonged painting work or in windy conditions

Quick Reference Table

Detailed Guidance

Masonry Preparation

Biological growth treatment: Algae, lichen, mould, and moss are common on UK masonry surfaces, particularly on shaded north-facing walls and in damp climates. Before any painting, all biological growth must be killed and removed. Apply a proprietary masonry fungicidal wash to the affected areas (and surrounding areas to prevent re-colonisation from the edges). Allow the stated dwell time (typically 24–48 hours) and then brush or wash off the dead growth. Never paint over live biological growth — it will continue to grow under the paint, which will blister and delaminate within months.

For persistent lichen (which is particularly resistant), a stronger biocide or a longer dwell time may be needed. Multiple treatments are sometimes required.

Power washing: After biocide treatment, power wash the entire surface at 100–150 bar to remove all loose and chalky material, dirt, efflorescence, and biocide residue. Allow the masonry to dry thoroughly before proceeding — this may take several days or weeks depending on weather. Do not paint onto damp masonry.

Stabilising solution: If the masonry surface is chalky (rubs off on your hand when touched) or friable, apply Sandtex Stabilising Solution or a proprietary equivalent. This is a clear penetrating primer that consolidates the surface and provides a sound bond for the topcoat. Apply liberally, allow to soak in, and apply a second coat. Allow to dry fully before painting. Without stabilising solution, paint applied to a chalky surface will peel because the bonding is only to the chalk layer, not to the underlying masonry.

New render pH: Portland cement has a pH of 12–13 when freshly mixed, which drops to approximately 8–9 over 6 months of weathering. Oil-based paints (including alkyd-based masonry paints) are attacked by highly alkaline surfaces — the alkali saponifies the binder, causing the paint to liquefy and run. If painting new render before 6 months of weathering, apply an alkali-resistant masonry primer or use a purely acrylic masonry paint (which is alkali-resistant) rather than an alkyd-based product.

Timber Preparation

Assessment for rot: Probe all timber sections with a penknife or thin screwdriver — if the blade sinks in easily (more than 3–4mm) without resistance, the wood is soft-rotted. All rotten timber must be cut out and replaced. Attempting to paint over rot, even with wood hardener, creates a job that will fail rapidly — the painted surface will look acceptable for 6–12 months before the continuing rot lifts the paint.

Back to bare: Strip existing paint where it has failed (peeling, bubbling, or where rot has been cut out) back to bare wood. Use heat gun, chemical stripper, or mechanical sanding as appropriate for the location and scale. For intricate profiles (window glazing bars, mouldings), chemical stripper is most precise. For flat surfaces, heat gun or power sanding is faster.

Knotting: Apply two coats of shellac-based knotting solution to all resinous knots before priming. Resin continues to bleed from knots for many years and will discolour, lift, and eventually break through paint if not sealed. Standard PVA or acrylic primers do not adequately seal knots — shellac knotting is specifically formulated for this purpose.

Primer selection by wood species: Not all timber requires the same primer. For most softwoods (pine, spruce, redwood), a conventional white or pink oil-based primer is appropriate. For high-tannin hardwoods (teak, iroko, oak, western red cedar), tannins bleed through oil-based primers and cause brown staining and adhesion problems. Use an aluminium wood primer, which seals the tannins, before any subsequent coats.

Metal Preparation

Surface preparation standard: The most important variable in metal painting is the surface profile achieved before priming. The BS 7079 series of standards (equivalent to ISO 8501) defines preparation grades from Sa 1 (light sweep blast) through Sa 3 (white metal blast). For maintenance painting of existing steelwork, St 2 (hand tool cleaning to bright metal) or St 3 (power tool cleaning to bright metal) is the practical standard. For new fabricated steel, Sa 2.5 (very thorough blast, leaving no more than 5% residual rust or mill scale staining) is the typical commercial requirement.

Rust treatment: Where it is not practical to remove all rust mechanically (complex fabrications, field joints, repairs in-situ), rust converter products (Jenolite, Hammerite Kurust, or similar) are applied to convert iron oxide to iron tannate, which is stable and can be painted over. These products are appropriate for light surface rust. On heavily corroded steel, they are not a substitute for mechanical preparation — use them only for residual surface rust after mechanical cleaning to the best achievable standard.

Primer selection: For ferrous steel, zinc phosphate primers (or other corrosion-inhibiting primers) have largely replaced traditional red oxide in professional applications. Zinc phosphate provides better adhesion to blast-cleaned surfaces and superior corrosion inhibition. Red oxide primer remains in common use for maintenance work and is still effective, but it is not the optimum specification.

For galvanised steel (zinc-coated), the zinc surface is chemically different from bare steel and conventional primers have poor adhesion to it. Use a proprietary galvanised metal primer (Rust-Oleum, Zinsser, or similar) or an etch primer to create a chemical bond to the zinc before applying the paint system.

COSHH Considerations for Lead Paint

Buildings constructed before approximately 1960 — and particularly pre-1940 — are very likely to contain lead paint in existing layers. In the UK, lead paint use in domestic properties was not formally prohibited until 1992, but awareness of health risks led to its reduction in use from the 1960s onwards.

Lead paint is not hazardous when in good condition and left undisturbed. It becomes a significant health hazard when sanded, scraped, heat-stripped, or power-ground — activities that create fine lead-containing dust or fumes. Lead in blood (blood lead level) above 10 μg/dL is associated with neurological harm, particularly in children.

COSHH assessment: Before any work on pre-1960 buildings involving abrading, stripping, or burning off paint, carry out (or commission) a COSHH assessment. Test the paint for lead content using a portable XRF analyser (available for hire) or by submitting a scraping sample to a laboratory. If lead is confirmed:

• Use wet methods to suppress dust (damp scraping, damp sanding)
• Use RPE rated FFP3 and eye protection
• Seal off the work area and prevent children and pregnant women from entering
• Bag and dispose of lead paint waste as controlled waste
• Consider whether encapsulation (overcoating without stripping) is viable — if the existing paint is sound, encapsulation with a suitable overcoat may be a safer option than stripping

Frequently Asked Questions

How long do I need to wait before painting new render?

For oil-based masonry paints (alkyd-based): at least 6 months for the pH of the cement to drop sufficiently. For acrylic-based masonry paints: many manufacturers state 28 days is sufficient (after the render has cured), but check the product datasheet — some still recommend 8–12 weeks. If painting earlier, apply an alkali-resistant masonry primer first.

Can I paint over existing masonry paint without stripping?

Yes, provided the existing paint is sound, well-bonded, clean, and not chalky. Clean thoroughly, treat any biological growth, and apply the new paint over the existing. If the existing paint is flaking or poorly bonded, strip back to a sound surface first. Different product types (smooth vs textured masonry paint) should not be mixed without checking compatibility.

What is the minimum number of coats for exterior timber joinery?

As a minimum specification: knotting solution (2 coats to knots) + wood primer (1–2 coats) + undercoat (1 coat) + topcoat gloss (2 coats). Total minimum 5–6 coats on bare timber. On previously painted timber in good condition: no primer needed; 1 coat undercoat + 2 coats topcoat is generally sufficient.

Do I need to prime galvanised guttering before painting?

Yes. Galvanised surfaces are notoriously difficult to paint without the correct primer. Clean with white spirit to remove any oils or contamination, lightly abrade with 180-grit wet-and-dry, and apply a proprietary galvanised metal primer before the topcoat. Some self-etching primers etch the zinc chemically and provide adhesion without abrasion.

Regulations & Standards

• COSHH Regulations 2002 — Risk assessment for lead paint, solvents, and biocides on site

• BS 7079 series — Surface preparation standards for steel substrates before painting (equivalent to ISO 8501)

• Work at Height Regulations 2005 — Scaffold and access equipment requirements for elevated external work

• HSE COSHH guidance on lead paint — EH40 occupational exposure limits; lead paint specific guidance on HSE website [verify current EH40 edition]

• HSE — Lead paint and construction workers — COSHH obligations for painters working with lead paint

• Sandtex Trade — Technical guidance — Stabilising solution and masonry paint specification

• Dulux Trade — Timber and metal painting guides — Exterior preparation system guidance

• BS 7079-A1 (ISO 8501-1) — Steel surface preparation grades

• HSE Work at Height Regulations guidance — Access equipment and scaffold requirements

• paint types guide — Choosing the right exterior paint product

• microporous coatings — Vapour-open coatings for exterior timber joinery

• specialist coatings — Intumescent and anti-carbonation coatings for structural applications