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Render vs Plaster — External & Internal Finishes
Understanding the difference between external render and internal plaster — materials, application, drying times, costs and when you need each.
Side-by-Side Comparison
| Property | Internal Plaster | External Render |
|---|---|---|
| Purpose | Smooth finish on internal walls/ceilings | Weather protection & finish on external walls |
| Base material | Gypsum (calcium sulphate) | Cement, lime, or proprietary polymer |
| Common products | Thistle Multi-Finish, Hardwall, Bonding | Sand & cement, K Rend, Weber, Parex |
| Typical thickness | 2-3mm (skim), 11-13mm (undercoat) | 15-20mm (two coats), 12-15mm (monocouche) |
| Number of coats | 1-2 coats | 2-3 coats (or 1 monocouche) |
| Setting time | 1-2 hours | 24 hrs between coats |
| Full drying time | 2-5 days | 7-28 days |
| Weather resistance | None — indoor use only | Designed for full weather exposure |
| Cost per m² (installed) | £15-25 | £40-80 |
| Typical lifespan | 30-50+ years | 20-40 years |
Pros and Cons
Internal Plaster
Pros
- + Quick setting time — same-day finishing
- + Smooth, paint-ready surface
- + Lower cost per m²
- + Easy to repair patches and cracks
- + No scaffolding needed
Cons
- - Cannot be used externally
- - Damaged by water and damp
- - Requires a sound, stable background
- - Short working time once mixed
- - Skill-dependent finish quality
External Render
Pros
- + Weatherproofs and protects external walls
- + Wide range of finishes and colours
- + Improves thermal performance
- + Transforms property appearance
- + Monocouche gives through-colour finish
Cons
- - More expensive than internal plaster
- - Scaffolding usually required
- - Weather-dependent application
- - Longer curing times (days to weeks)
- - Sand/cement render can crack over time
UK Cost Comparison (2026 Prices)
| Finish Type | Materials per m² | Labour per m² | Total per m² |
|---|---|---|---|
| Skim plaster (internal) | £2-4 | £12-20 | £15-25 |
| Undercoat + skim (internal) | £4-7 | £18-28 | £22-35 |
| Sand & cement render (external) | £5-10 | £30-50 | £40-60 |
| Monocouche render (external) | £12-18 | £35-55 | £50-75 |
| Silicone render system (external) | £15-22 | £40-60 | £55-80 |
Note: external render prices include scaffolding. Internal plaster prices assume plasterboard or sound masonry background.
When to Choose Each
You Need Plaster When
- Finishing internal walls or ceilings
- Creating a smooth surface for painting
- Skim coating over plasterboard
- Patching damage inside the property
You Need Render When
- Weatherproofing external walls
- Changing the appearance of brickwork
- Part of an external wall insulation system
- Covering damaged or mismatched external masonry
Related Calculators
- Rendering Calculator — calculate sand and cement for external render
- Plaster Coverage Calculator — bags of plaster for internal walls
Next up: related pages
How We Calculate This
Frequently Asked Questions
No. Internal plasters like Thistle Multi-Finish or British Gypsum bonding coat are gypsum-based and will dissolve in rain. External walls need cement-based render, lime render, or proprietary systems like K Rend monocouche which are designed to withstand weather. To estimate bags, coverage and cost per m², see our K Rend monocouche render calculator.
Internal gypsum plaster sets within 1-2 hours and is fully dry in 2-5 days depending on conditions. External cement render needs 24 hours between coats and 7-28 days to fully cure. Monocouche render is typically rain-resistant within 24 hours.
Yes. Rendering costs £40-80/m² for a two-coat sand and cement system vs £15-25/m² for internal skim plastering. The higher cost reflects weather-resistant materials, scaffolding requirements, and the need for multiple coats with curing time between each.
You should not render over internal plaster to create an external finish. For internal walls, you can plaster over old plaster if it is sound — apply PVA or SBR bonding agent first. For external walls, loose or blown render should be hacked off and replaced, not patched over with plaster.
These are different skills. Internal plastering (skim coating, dot and dab) is done by plasterers. External rendering (sand and cement, monocouche, silicone) is done by renderers. Some tradespeople do both, but many specialise. Always check that your tradesperson has experience with the specific finish you need.
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Last updated: April 2026
Verified against UK standards · estimates only, confirm with your supplier.