Can You Use Emulsion As Undercoat? (The Definitive Guide)

White emulsion paint can be used as an undercoat for interior wood and plaster surfaces. Dilute the emulsion with tap water in a 3:1 ratio and apply a smooth, even layer over a prepared surface. Let the emulsion dry for at least 2 hours before overpainting; only use water-based paints over an emulsion undercoat – latex and oil-based paints are unsuitable and prone to peeling.

The ideal undercoat acts as a perfectly smooth, level layer that forms a strong bond between a surface and the layers of paint applied over it.

Getting the undercoat right is vital when painting any large surface, particularly plastered walls and ceilings. Any imperfections in the undercoat can stick out like a sore thumb, even after overpainting with multiple layers.

An emulsion is a type of water-based paint – it’s generally used to paint walls and ceilings.

Emulsions contain added chemicals which increase their stability and durability. These chemicals are typically a mixture of synthetic resins, coloured pigments, biocides and other chemical additives.

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Emulsion paints aren’t designed for use as an undercoat; however, they can form a smooth and durable undercoat on some indoor surfaces.

Read on to find out which materials are suitable for an emulsion undercoat. We’ve also included the steps you need to take to correctly thin an emulsion for undercoat painting.

What’s The Difference Between Emulsion And Undercoat?

The term emulsion refers to a specific type of water-based paint.

An undercoat refers to any smooth layer applied to a surface to be further painted or varnished with additional layers. The main function of an undercoat is to create an easy-to-paint surface for further applications of paint or finish.

Most emulsions available from DIY shops, supermarkets and paint shops are labelled for indoor use only and are rarely marketed for use as an undercoat.

However, emulsions can be used as a safe and effective undercoat on some indoor surfaces. See the table below for guidance on when an emulsion undercoat may be suitable.

Emulsions that contain synthetic resins (acrylic, vinyl, acetate or a mixture of these substances) will last longer than those without. These chemical additives help to prevent scratches and resist peeling.

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Which Emulsion Paint Is Best?

Using a high-quality, premium emulsion will produce a noticeably better finish. These products can be relatively expensive, but it’s worth spending the extra cash and investing in the best brands.

Low-quality emulsions have a thicker, glue-like consistency. Painting a smooth and even finish with these emulsions isn’t impossible, but it will take up more of your time and effort.

They may also contain less useful chemical additives, degrading any overcoats more quickly.

Cheaper emulsions can also contain less biocide. A biocide is mixed into almost all paints – they are specialised chemicals that prevent mould from forming in the paint tin and on painted walls.

Always follow the manufacturer’s guidance when using emulsions. Pay close attention to the required drying time between coats. This can range from 30 minutes to many hours; the drying time will be longer in cold and humid environments.

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Can Emulsion Paint Be Used As A Primer?

Absolutely! Emulsions can be used as a primer, but there are additional steps you’ll need to take.

A primer stops wood, plaster and other porous surfaces from absorbing paint or other overcoats. The primer acts as a protective layer between the material and any overcoat layers, such as matte, gloss or satin paints.

Overcoats are sometimes referred to as topcoats or finishing coats.

Primers help paint bind with plaster that’s overly wet, soft or powdery. They also act as a physical barrier between plaster and paint; plaster is alkaline and may chemically react with some types of paint, leaving you with unsightly and unstable walls and ceilings.

All materials, except metal, are porous; this means they trap small amounts of moisture within their internal structure.

When the temperature rises, such as in the summer months or in a heated room, this moisture will slowly leach out of the material and into the surrounding atmosphere, cracking through layers of paint in the process.

How Much Water Do You Mix With Primer?

To use emulsion as a primer, increase the water content of your emulsion by thinning it with tap water. This is referred to in the trade as adding a mist coat.

Dilute your emulsion paint by adding one part water to three parts paint, or until thin enough to barely stick to a paintbrush. The goal is to add a very thin layer of primer to your surface, to prepare it for painting.

When Can’t I Use Emulsion As An Undercoat?

If you’re planning on painting a wall or ceiling with oil-based, latex-based or enamel paints, don’t use an emulsion undercoat. As the old saying goes, oil and water don’t mix and this advice should be adhered to when painting.

Only use water-based paints, such as acrylic, on top of an emulsion undercoat. Oil-based paints cannot physically bond with water-based emulsions.

If you choose to ignore this crucial information, you’ll be left with a rubbery, peeling mess that only gets worse as the layers of paint continue to dry!

Unfortunately, the only way to remedy this mistake is to scrub away your hard work and start again from square one. Click here to find out how to remove emulsion paint from plaster.

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Can You Use Emulsion As Undercoat On Wood?

Emulsions can be used to undercoat wooden surfaces but they must be used on indoor surfaces only.

Most emulsion paints aren’t durable enough to withstand exterior applications – they’re not weather resistant and can’t withstand extreme fluctuations in temperature.

Satinwood paint is almost always a more resilient choice for undercoating and painting wood. However, these items are often more expensive than emulsions, which can work just as well for a fraction of the cost.

Wooden floors should not be painted with emulsion – they’re simply too weak.

Wooden skirting, bannisters, walls, ceilings, furniture and other surfaces that aren’t frequently touched are suitable for emulsion undercoating and overcoating.

Wood that’s been coated in an emulsion undercoat will have lower durability than plaster and plasterboard.

To increase the emulsion’s grip, sand the wooden surface thoroughly before applying an undercoat. A layer of varnish or an overcoat with thicker emulsion is essential if you want your painted wood to last.

Is Primer Necessary For Painting Walls?

All porous surfaces should be primed before painting.

Without a primer, even trace amounts of moisture will be forced through any painted overcoat surfaces as it tries to escape the material.

Eventually, this will cause your overcoated surface to crack, peel or bubble. This expensive and time-consuming mistake can be easily remedied by choosing the correct primer before applying subsequent layers of paint.

There are a wide variety of specialised primer paints available for different materials, but standard emulsion can be used. White emulsions are best, as coloured emulsions contain pigments which can prevent the emulsion from correctly bonding to a surface.

Can I Use An Emulsion Undercoat On…?

Below is a list of different materials you may want to apply your emulsion to and the whether or not you can.