Exterior Paint Preparation Guide: How to Prep Your House for Painting

Why Surface Preparation Makes or Breaks Exterior Paint

Ask any professional painter what determines how long an exterior paint job lasts, and the answer is always the same: preparation. A flawless topcoat applied over a poorly prepared surface will fail within 1-2 years — peeling, blistering, and flaking off in sheets. The same paint, applied over a properly cleaned, repaired, and primed surface, will last 5-8 years or more.

This guide walks through every stage of exterior surface preparation, from initial assessment to the final primer coat, with specific instructions for each surface type.

Preparation Steps by Surface Type

Every exterior surface needs attention, but the preparation steps differ. Use this table to plan your workflow.

Step 1: Assess the Surface

Before you pick up a pressure washer, walk around the entire house and assess the current condition. Take notes or photos of:

• Flaking or peeling paint — Mark areas where paint is coming away. These need scraping back to a firm edge.
• Cracks — Note hairline cracks (cosmetic) versus wider cracks (potentially structural). Cracks above windows and doors or in stepped diagonal patterns may indicate subsidence.
• Mould, algae, or lichen — Common on north-facing walls, under gutters, and near trees. These must be treated before painting.
• Damp patches — Investigate the cause (leaking gutter, rising damp, blocked drainage) and fix it before painting. Paint will not solve a damp problem.
• Bare patches — Areas where the surface is exposed will need primer.
• Chalky or powdery surfaces — Rub your hand across the wall. If white powder comes off, the surface needs stabilising solution before painting.

Step 2: Pressure Washing

Pressure washing removes years of dirt, algae, loose paint, and surface contaminants that prevent paint adhesion. It is the most important single preparation step.

Equipment needed: A pressure washer with 100-130 bar output and a fan nozzle (25-degree or 40-degree). Avoid turbo or zero-degree nozzles on render and wood — they can gouge the surface.

Technique: Work from top to bottom, holding the nozzle 30-50 cm from the wall. Overlap each pass by about a third. On textured surfaces like pebbledash, use a wider nozzle angle and lower pressure to avoid dislodging aggregate.

Safety: Wear safety goggles and waterproof clothing. Keep the jet away from window seals, vents, and damaged mortar joints where water could penetrate behind the wall.

Drying time: Allow a minimum of 48 hours in warm, dry weather. In cooler or humid conditions, allow 3-5 days. The wall must be completely dry before any filler, primer, or paint is applied.

Step 3: Scraping and Sanding

Once the wall is dry, return to the areas you identified with loose or flaking paint.

• Use a stiff scraper or filling knife to remove all loose paint. Scrape until you reach firmly adhered paint or bare surface.
• Feather the edges where old paint meets bare surface using 80-grit sandpaper. This prevents visible ridges showing through the new paint.
• On wood cladding, sand the entire surface lightly with 120-grit to provide a key for the new paint.
• Remove all dust with a brush or dry cloth before the next step.

Step 4: Treating Mould, Algae, and Lichen

Biological growth is one of the most common issues on exterior walls, especially on north-facing and shaded surfaces.

Treatment method: Apply a proprietary fungicidal wash (available from any DIY store, typically EUR 8-15 per litre) with a brush or sprayer. Leave it on for the time specified on the product (usually 24-48 hours). The mould will die and turn white or grey. Scrub off the dead growth with a stiff brush and rinse with clean water.

Do not just pressure wash mould off — this removes the visible growth but leaves spores in the surface. Without chemical treatment, mould will return within months.

Step 5: Filling Cracks and Repairs

Cracks allow water to penetrate behind the paint film, causing blistering and peeling from behind. Filling every crack before painting is essential.

Hairline cracks (under 3 mm): Use a flexible exterior filler applied with a filling knife. Slightly overfill and sand flush when dry.

Medium cracks (3-10 mm): Use exterior caulk or mastic from a cartridge gun. Press the caulk deep into the crack and smooth with a wet finger or caulk tool.

Large cracks or missing render (over 10 mm): Repair with exterior render repair mortar. For brick walls, repoint damaged mortar joints with matching mortar. Allow render repairs to cure for a minimum of 7 days before painting.

Wood repairs: Fill rotted or damaged areas with two-part epoxy wood filler. For larger sections, cut out the damaged timber and splice in new wood.

Step 6: Stabilising and Priming

Priming is the bridge between the prepared surface and the topcoat. It serves three purposes: sealing porous surfaces, improving adhesion, and providing a consistent base colour.

When to use stabilising solution: Apply to any surface that is chalky, powdery, or excessively porous. Stabiliser penetrates and binds the loose surface layer, creating a firm base for primer.

When to use masonry primer: Apply to all bare render, bare brick, filled areas, and any surface where the old paint has been completely removed. Primer is not needed over sound, existing masonry paint.

When to use wood primer: Apply to all bare wood, filled areas, and sanded surfaces on timber cladding. Use an aluminium-based primer on knotty wood to prevent resin bleed-through.

Drying Times by Weather Conditions

Temperature and humidity dramatically affect drying times for fillers, primers, and paint. Rushing these intervals is a common cause of premature failure.

As a rule, do not start or continue exterior painting when the temperature is below 10°C, when rain is forecast within 4 hours, or when relative humidity exceeds 85%.

Weather Planning for Exterior Painting

The weather window is the most unpredictable part of any exterior painting project. Plan carefully:

• Ideal conditions: 15-25°C, low humidity, light or no wind, no rain forecast for at least 8 hours.
• Acceptable conditions: 10-30°C, moderate humidity, light breeze (helps drying), dry forecast for 4+ hours.
• Avoid: Temperatures below 10°C, direct blazing sun on the wall (paint dries too fast and leaves brush marks), fog, mist, heavy wind (carries debris into wet paint), and any rain risk.

Start painting the side of the house that will be in shade during the hottest part of the day. On south-facing walls, start early morning and stop before the sun hits the surface directly.

Preparation Cost Breakdown

Proper preparation does add cost, but it is money well spent given the extended paint life.

For most houses in reasonable condition, preparation materials cost EUR 100-200. If the surface is in poor condition with extensive flaking, mould, and cracks, budget up to EUR 250-300 for materials plus significantly more time.

Preparation Checklist

Use this checklist to ensure you have completed every preparation step before opening the first tin of topcoat:

1. Full exterior inspection completed, problem areas marked
2. Gutters, downpipes, and drainage checked and repaired
3. All surfaces pressure washed and dried (48+ hours)
4. Mould and algae treated with fungicidal wash
5. Loose and flaking paint scraped and sanded smooth
6. All cracks filled with appropriate filler or caulk
7. Render repairs and repointing completed (7+ days curing)
8. Chalky surfaces treated with stabilising solution
9. All bare surfaces primed with appropriate primer
10. Masking applied to windows, doors, and surfaces not being painted
11. Weather forecast checked — 2+ dry days ahead
12. Drop sheets and ground protection in place