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

> Complete guide to preparing exterior walls for painting. Learn power washing, scraping, filling, priming, and weather timing for a lasting finish.

Canonical: https://renoquant.com/exterior-paint-prep-guide
Prices updated: 2026-03

## 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.

| Prep Step | Smooth Render | Rough Render / Pebbledash | Brick | Wood Cladding |
|---|---|---|---|---|
| Pressure wash | Yes — medium pressure (100-120 bar) | Yes — low-medium pressure (80-100 bar) | Yes — medium pressure (100-130 bar) | Yes — low pressure (60-80 bar) |
| Scrape loose paint | Yes — scraper and wire brush | Yes — wire brush only (avoid damaging texture) | Yes — wire brush | Yes — scraper and sandpaper |
| Treat mould/algae | Fungicidal wash if present | Fungicidal wash if present | Fungicidal wash if present | Fungicidal wash + wood preserver |
| Fill cracks | Exterior flexible filler | Exterior filler (match texture) | Repoint mortar joints | Wood filler or epoxy filler |
| Sand edges | 80-grit to feather edges | Light sand only | Not needed | 120-grit for smooth finish |
| Apply stabiliser | If surface is chalky/powdery | If surface is chalky/powdery | Always on bare brick | Not needed (use wood primer) |
| Prime bare patches | Masonry primer | Masonry primer | Masonry primer | Wood primer/undercoat |
| Drying time before topcoat | 24 hours after primer | 24 hours after primer | 24 hours after primer | 16-24 hours after primer |

### 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.

| Product | 20°C, Low Humidity | 15°C, Moderate Humidity | 10°C, High Humidity | Below 10°C |
|---|---|---|---|---|
| Exterior filler | 2-4 hours | 4-6 hours | 6-10 hours | Not recommended |
| Stabilising solution | 4-6 hours | 6-8 hours | 10-16 hours | Not recommended |
| Masonry primer | 4-6 hours | 6-10 hours | 12-16 hours | Not recommended |
| Wood primer | 4-6 hours | 6-8 hours | 10-14 hours | Not recommended |
| Masonry topcoat (between coats) | 4-6 hours | 6-10 hours | 12-24 hours | Not recommended |
| Masonry topcoat (full cure) | 7 days | 10-14 days | 14-21 days | Not recommended |

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.

| Item | Approximate Cost (EUR) | Covers |
|---|---|---|
| Pressure washer hire (1 day) | 30-50 | Full house exterior |
| Fungicidal wash (5L) | 12-20 | 30-50 m² |
| Exterior flexible filler (1L tube) | 8-15 | 10-20 linear metres of cracks |
| Exterior caulk cartridge | 4-8 | 8-12 linear metres |
| Masonry stabiliser (5L) | 15-25 | 20-30 m² |
| Masonry primer (5L) | 20-35 | 30-40 m² |
| Sandpaper (assorted pack) | 5-10 | Multiple uses |
| Wire brush & scraper set | 10-20 | Reusable |
| **Total preparation materials** | **EUR 104-183** | **Typical semi-detached house** |

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

## Frequently Asked Questions

### How long should you wait after pressure washing before painting?

Wait at least 48 hours after pressure washing before painting, and longer if the weather is cool or humid. Masonry must be completely dry throughout — not just surface-dry. Test by taping a plastic sheet to the wall overnight; if condensation forms underneath, the wall is still too wet.

### Do I need to remove all old paint before repainting?

No. Only remove paint that is flaking, peeling, or blistering. Sound, well-adhered old paint provides a good base for new paint. Scrape loose areas back to a firm edge, feather the edges with sandpaper, fill any depressions, and prime bare patches before recoating.

### What should I use to fill cracks in exterior walls?

Use flexible exterior filler for hairline cracks up to 5 mm wide. For larger structural cracks (5-15 mm), use exterior-grade caulk or mastic. Cracks wider than 15 mm or those that are actively growing should be assessed by a structural engineer before any cosmetic repair.

### Can I paint over mould on exterior walls?

Never paint over mould or algae. First, treat the affected area with a fungicidal wash and leave it for 24-48 hours. Scrub off dead mould with a stiff brush, rinse, and let the wall dry fully. Then apply a fungicidal primer before your topcoat. Painting over mould traps moisture and the mould will grow through the new paint.

### How do I prepare bare brick for painting?

Clean the brick with a pressure washer to remove dirt and loose mortar. Repair any damaged pointing with matching mortar and let it cure for at least 7 days. Apply a masonry stabilising primer to seal the porous surface and improve adhesion. Two coats of masonry paint on primed brick will give a solid, long-lasting finish.

### Is exterior paint preparation really necessary?

Proper preparation is the single most important factor in how long exterior paint lasts. Paint applied to a dirty, damp, or flaking surface will peel within 1-2 years. The same paint on a clean, dry, properly prepared surface will last 5-8 years. Preparation typically accounts for 50-70% of a professional painter's time on an exterior job.

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Interactive version with calculator: https://renoquant.com/exterior-paint-prep-guide. Figures are estimates; final quotes vary by site conditions, materials, and region.