Interior vs Exterior Coating Systems — Spec and Quantity
Interior and exterior painting are priced separately because the systems, the wastage, and the labour productivity are genuinely different.
System comparison
| Interior masonry/plaster | Exterior masonry | |
|---|---|---|
| Sealer/primer | 1 coat alkali-resisting | 1 coat penetrating masonry sealer (heavier) |
| Finish | 2 coats emulsion (matt/silk) | 2 coats 100% acrylic / weathershield, or 1 coat elastomeric |
| Key properties | washability, low odour | UV stability, water resistance, dirt pick-up resistance, flexibility |
Quantity differences
- Wastage: interior 5–8%; exterior 8–15% (wind, scaffolding, spray drift, touch-ups).
- Coverage: exterior products are often thicker-bodied → lower m²/L.
- Access: exterior needs scaffold, swing stage, or boom — a separate preliminaries cost that dwarfs the paint on tall buildings.
Elastomeric coatings
High-build elastomeric coatings (200–400 µm) bridge hairline cracks in exterior masonry and cut water ingress. They cost more per litre, cover far fewer m²/L, and need a compatible flexible sealer — model them as their own system, not a finish-coat substitution.
Weather and productivity
Exterior painting stops in rain and in high humidity or low temperature outside the product window. In the Philippine wet season this compresses the workable window — reflect it in the programme and the labour rate, not just the material.
EngEst Pro’s Painting Calculator keeps interior and exterior schedules separate, applies the right system and wastage to each, and lets you switch an exterior wall to an elastomeric system with the quantities and price recalculated.
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