Wall Paints: What Changes Between Interior and Exterior
Applying an interior paint to an exterior wall means redoing the job in 12 months. Products differ in resins, fillers, and additives. The choice of the correct paint starts with the environment: indoors, washability is sought, outdoors, resistance to UV and thermal shock.
The Chemical Basics: Acrylic, Vinyl, Silicone, Silicate
Each binder has different properties. Acrylic is elastic and provides good coverage. Vinyl is cheaper but doesn't last long. Silicone is water-repellent without closing pores. Silicate chemically bonds with the substrate and does not form a film.
- Acrylic – washable, flexible, resistant. Suitable for interiors and exteriors on stable substrates.
- Vinyl – economical, low breathability. Only for dry interiors and ceilings.
- Siloxane – water-repellent and breathable. For exteriors and interiors with condensation.
- Potassium Silicate – very high breathability, mineral bond. For masonry restoration and historical buildings.
Technical Tip: On walls with rising damp problems, avoid acrylics. Use only silicate paints: they do not retain water and allow the wall to breathe. The cost per m² is higher, but the problem will not recur.
Which Paint for Each Room: Washability and Resistance Data
The UNI 10560 standard classifies paints based on washing cycles. The higher the number, the more resistant it is to sponging. Class 1 is needed in hallways and kitchens; class 2 is sufficient in bedrooms.
| Environment | Minimum Class | Recommended Paint Type | Notes |
|---|---|---|---|
| Kitchen | Class 1 | Washable acrylic with anti-grease additives | Resistant to steam and splashes. Choose satin, not matte. |
| Bathroom | Class 1 | Acrylic or siloxane with anti-mold | Medium breathability. Avoid vinyls. |
| Hallway | Class 1 | Washable acrylic | High traffic, gets dirty easily. |
| Bedroom | Class 2 | Matte acrylic or siloxane | Low wear, class 3 can also be used on ceilings. |
| Living Room | Class 2 | Matte acrylic | Medium resistance, better Class 1 if there are children. |
| Ceilings | Class 3 | Vinyl or economical acrylic | Not touched, resistance not necessary. |
Exterior Paints: How They Last Over Time
Outdoors, resistance to UV rays, rain, and frost is measured. A poor quality paint loses color after 2 years and peels after 5. Real durability data helps in choosing.
Average Durability by Exterior Paint Type
| Paint Type | Average Durability | UV Resistance | Breathability | Indicative Cost (€/m²) |
|---|---|---|---|---|
| Pure Acrylic | 5–7 years | Medium | Low | 2–4 € |
| Siloxane | 8–10 years | High | High | 4–6 € |
| Potassium Silicate | 10–15 years | Very High | Very High | 6–9 € |
| Hydraulic Lime | 8–12 years | High | Very High | 5–8 € |
Prices refer to material only. On south-facing surfaces or on the Tyrrhenian coast, siloxane or silicate paints are better: UV rays degrade acrylics in 5 years.
How to Calculate the Amount of Paint Needed
Miscalculating means stopping work or ending up with open cans that dry out. The theoretical yield is written on the can, but the substrate changes everything.
- Measure the area: wall length × height. Subtract doors and windows (approx. 2 m² per door, 1.5 m² per window).
- Check the m²/l yield indicated on the package (on average 8–12 m²/l).
- Divide the area by the yield, multiply by the number of coats (2 or 3).
- Add 10% for waste and touch-ups.
- Always round up to the nearest liter.
Technical Tip: On new or rough plaster, the yield decreases by 20–30%. If the plaster is very absorbent, apply a primer first: this reduces paint consumption by 15% and unifies the finish.
Actual Yield Based on Substrate
| Substrate | Average Yield (m²/l) | Coats Needed | Required Base Coat |
|---|---|---|---|
| New civil plaster | 10–12 | 2 | Primer if absorbent |
| Rough/rustic plaster | 7–9 | 2–3 | Always primer |
| Joint-compound plasterboard | 8–10 | 2 | Insulating primer |
| Existing paint (same color) | 11–13 | 1–2 | Sanding if glossy |
| Existing paint (color change) | 9–11 | 2–3 | Depends on contrast |
| Exposed concrete | 6–8 | 2–3 | Adhesion primer |
From BuildZone
At Rosignano Solvay, we keep a permanent stock of professional paints: washable acrylics class 1, exterior siloxanes, pure silicates, and primers. If you are preparing a quote, send us the list of surfaces via WhatsApp at +39 0586 764566: we will calculate the exact liters and check availability within the day.
Conclusion
Choosing the correct paint is based on technical data: washability class, breathability, yield. Interior and exterior require different products. Have you already checked the type of substrate you need to work on, or do you need to compare two products?
