Complete Guide to Surface Preparation Before Painting

The Foundation Principle

A premium paint applied over a poorly prepared surface will fail. A basic paint applied over a perfectly prepared surface will perform far beyond its specification. Surface preparation is not a step before painting — it is the most critical phase of the entire process.

Stage 1: Surface Assessment

Before any preparation begins, conduct a systematic assessment of the surface:

For New Plaster/Concrete: - **Age check:** New plaster must cure for minimum 28 days before painting. Painting over uncured plaster traps alkali salts beneath the film, causing early failure. - **Moisture test:** Tape a 500mm × 500mm sheet of polyethylene to the wall and leave for 24 hours. Condensation on the underside indicates moisture content too high for painting (above 15% moisture content). - **Soundness:** Tap the surface systematically with a wooden mallet. Hollow sounds indicate areas of poor adhesion between plaster and substrate — these must be removed and replastered.

For Previously Painted Surfaces: - **Adhesion test:** Press masking tape firmly onto the existing paint, then pull sharply. If paint lifts with the tape, the existing coating has failed adhesion and must be removed entirely. - **Alkali/salts:** White powdery deposits (efflorescence) indicate active salt migration — the source of moisture driving the salts must be addressed before repainting. - **Chalking:** Rub your palm across the surface. Chalky transfer indicates the existing paint has oxidised beyond its service life and should be cleaned off before recoating.

Stage 2: Crack Assessment and Treatment

Classify cracks before treating them:

Hairline cracks (< 0.3mm): Apply wall putty. These are cosmetic and can be bridged by an elastomeric topcoat.

Fine cracks (0.3–1mm): Route out to a V-profile using an angle grinder, vacuum clean, and fill with an acrylic wall filler. Allow full cure before sanding.

Wide cracks (1–5mm): V-rout, prime with PVA adhesive, fill with polymer-modified mortar. Check for structural movement causes — if the crack is recurrent, structural advice may be needed.

Structural cracks (> 5mm or active): Do not attempt to cosmetically fill these. Engage a structural engineer to assess and rectify the underlying cause before any surface treatment.

Stage 3: Cleaning

Interior Walls: 1. Remove all dust with a dry cloth, working systematically from top to bottom. 2. Wash down with a solution of 1 part liquid detergent to 10 parts water, rinsing thoroughly. 3. Treat any mould or algae with a fungicidal wash (diluted bleach at 1:5 ratio). Allow to kill the mould completely — minimum 24 hours. 4. Rinse and allow to dry completely.

Exterior Walls: 1. Pressure wash at 1,500–2,000 PSI to remove loose paint, dirt, algae, and chalking. 2. Allow minimum 48 hours drying after pressure washing. 3. Treat persistent algae/fungal growth with a diluted biocidal wash.

Stage 4: Sanding

Sanding creates a mechanical key (micro-profile) that dramatically improves adhesion of subsequent coats.

• **New plaster over putty:** Sand with 120 grit after each putty coat, then 180 grit on the final coat before priming.
• **Previously painted surfaces:** Sand entire area with 100–120 grit to remove surface gloss and improve adhesion.
• **Woodwork:** Sand progressively from 80 to 120 to 180 grit, working with the grain.

Always vacuum and wipe down thoroughly after sanding — dust contamination prevents adhesion.

Stage 5: Putty Application

Wall putty fills micro-roughness and hair cracks to create a smooth, uniform base:

1. Mix water-based acrylic putty to a smooth, creamy consistency. 2. Apply first coat with a steel putty blade at approximately 1–1.5mm thickness. 3. Allow to dry completely (6–8 hours minimum). 4. Sand lightly with 120 grit and remove all dust. 5. Apply second coat to fill remaining imperfections. 6. Sand final coat with 180 grit for a glassy smooth surface.

Stage 6: Priming

The primer creates the critical bond between the substrate and the decorative topcoats:

• **New plaster:** Alkali-resistant water-based primer, diluted 25–30%. One coat.
• **Wood (interior):** Wood primer, lightly sanded after drying.
• **Metal:** Appropriate metal primer (zinc phosphate for steel, etch primer for aluminium).
• **Over putty:** Interior wall primer, one coat, diluted 20%.

The Professional Standard

At G.G. Global Enterprise, our surface preparation is governed by a written QC checklist verified by our site supervisor before any topcoats are applied. We never compromise on preparation time — it is the single factor most within our control that determines the longevity of our work.