The core difference lies in their typical use and application stage. Emulsion paint is primarily a decorative finish for walls and ceilings, while an undercoat is a preparatory layer applied after priming or painting, but before the final topcoat.
Understanding Emulsion Paint
Emulsion is the most common type of paint used for interior walls and ceilings. It's water-based and relatively quick-drying.
- Primary Use: Decorative finish coat on interior walls and ceilings.
- As a Primer: As highlighted in the provided reference: "On fresh plaster, primer is normally an emulsion paint mixed with 10 per cent water – this soaks into new plaster, reducing its absorbency so new paint sticks to the wall." This shows thinned emulsion can serve as a specific type of primer (a 'mist coat') on new, absorbent surfaces like plaster.
Understanding Undercoat
An undercoat is a paint layer applied after the surface has been prepared (cleaned, filled, sanded) and often after a primer has been used, but before the final layer of paint (the topcoat).
- Primary Use: To provide a stable base, improve adhesion for the topcoat, enhance colour coverage, and sometimes block stains.
- Application Stage: Crucially, the reference states: "An undercoat is used on a surface that's been painted or primed." This means it's applied over an existing layer, not directly onto a raw, unprimed surface (unless the product specifically states it's a self-priming undercoat).
Key Distinctions: Emulsion vs. Undercoat
Here's a quick summary of the main differences:
| Feature | Emulsion Paint | Undercoat Paint |
|---|---|---|
| Primary Role | Decorative finish | Preparatory layer for topcoat |
| Application | Applied as a final layer or, if thinned, as a primer (on fresh plaster) | Applied over existing paint or primer |
| Purpose | Colour and finish | Adhesion, coverage, hiding previous colour/stains, creating smooth base |
| Typical Use | Walls and ceilings | Wood, metal, previously painted surfaces before applying topcoat |
Why Use Undercoat After Priming/Painting?
Even when a surface has been primed or previously painted, an undercoat offers specific benefits before applying the final decorative layer (like gloss or satinwood on woodwork, or sometimes even a strong colour emulsion over a dark base):
- Improved Adhesion: Creates a surface that the topcoat can stick to better.
- Enhanced Opacity: Helps the topcoat cover the previous colour or surface imperfections more effectively.
- Uniform Base: Provides a consistent surface, especially useful when changing colours drastically or painting over repairs.
- Durability: Contributes to the overall lifespan and finish of the painted surface.
In simple terms, think of it as building layers: Primer prepares the raw surface, Undercoat perfects the base for the topcoat, and Emulsion (or other topcoat) provides the final look.
