What is a Mist Coat?
A mist coat is a diluted first coat of paint applied to new plaster or very porous surfaces. The primary purpose is to seal the surface and reduce its absorbency.
When painting freshly plastered walls, applying a mist coat is essential. This initial coat will help seal the plaster, reducing its absorbency, ensuring a better adhesion and a smoother finish for subsequent coats.
Why is the Ratio Important?
A mist coat that’s too thin may fail to provide adequate coverage and sealing. Conversely, a coat that’s too thick might sit on the surface without properly adhering, risking future peeling or flaking.
Manufacturer Recommendations
The first point of reference for a mist coat ratio should be consulting the paint manufacturer’s recommendations. Here are some examples:
- Leyland Contract Matt: Often suggests approximately 10% water dilution (9 parts paint to 1 part water).
- Dulux Trade: Commonly advises a 25% water dilution.
A commonly seen recommendation is to use a ratio of 50/50 paint-to-water; however, this extreme dilution has drawbacks:
- The mix can be overly thin, soaking immediately into the plaster, possibly causing patchiness.
- Achieving opacity might be difficult, requiring more subsequent coats.
Common Ratios and Considerations
Commonly recommended mist coat ratios range from 70/30 to 80/20 (paint to water). Factors influencing your choice include:
- Emulsion Type: Non-vinyl matt emulsions (often labeled as “contract matt”) are recommended, as vinyl-based paints don’t penetrate plaster as effectively.
- Plaster Porosity: Very porous or dry plaster may require a slightly higher dilution for deeper penetration.
Best Application Practices
- Thorough Mixing – Ensure the paint and water mixture is uniform. Hand mixing with a plastic spatula can help achieve consistency.
- Even Application – Apply generously but evenly using a roller. Work quickly to blend edges and prevent visible overlap lines.
- Accept Some Imperfections – Initial mist coats often show slight irregularities. These imperfections can be rectified and then resolved in subsequent coats (see below).
- Avoid Premature Cleaning – Do not wipe freshly painted mist coats with damp cloths this can remove the thin layer.
- Fix Imperfections After Drying – Fill and sand imperfections highlighted by the mist coat. Once dry, reapply the mist coat to the repaired areas.
When to consider a second Mist Coat
If the initial coat looks patchy or the plaster is extremely absorbent, a second mist coat (possibly slightly less diluted) can improve the uniformity and effectiveness of sealing. If imperfections revealed by the first coat have been filled and sanded, a second mist coat or a partial second mist coat will help provide a good base for further painting.