The Ultimate Guide to Primer Paint

With any painting job, preparing your surface correctly is key to a smooth, professional, and long-lasting finish. Primer paint is a vital step when preparing your surface.

Buying the right primer for your job can be a daunting prospect. You’re in the DIY store and are confronted by endless isles of tins, all looking frustratingly similar. With any painting job, preparing your surface correctly is key to a smooth, professional, and long-lasting finish. Primer paint is a vital step when preparing your surface.

Whether you’re covering a dark colour on interior walls, painting wooden doors, or adding paint to your metal surfaces, it can be tempting to get your new colour on straight away.

Using a primer paint will make a significant difference to the standard of your finished paint job and is worth the extra time and effort. As well as a professional finish, using a primer paint has practical benefits too, such as preventing stains and neutralising odours.  So, set your tin of topcoat paint aside for now and read on to find the right primer paint for your task.

A finish as good as this simply isn’t possible without using a primer paint.

What Is Primer Paint and What Does It Do?

Primer paint is the first layer of paint you will apply to your surface. If you are painting wood or metal, you will have already prepared your surface by sanding and descaling etc. Primer paint should be the first stage of your coating and paint system.

It can be tempting to skip the preparation stage when you’re keen to see the finished product and are itching to add that bold colour-pop to your surface.

However, using a primer is almost always worth the extra effort. The primer adds a layer between your raw surface and subsequent coats of paint. On porous surfaces, this smooths out the surface, avoiding the need for unnecessary coats of paint later and an uneven final result.

Primer paint also helps your undercoat and topcoat to adhere to the surface you’re working on.

Although you can use a roller or spray gun to apply your primer, it’s best applied with a brush. You have greater control over the amount of product you are using and can easily feel dry patches or areas where the primer may be too thick as you go.

Different Types of Wood Primer

A wooden staircase that was primed using Johnstone’s Wood Primer followed by an eggshell top coat.

If you are painting wood, you will need to use a primer before you start painting. Wood, especially softwood, is very porous, so using a primer will stop your paint from soaking into your surface. There are several different types of primer for wood, so read on to find the right one for your wood painting project.

Water-borne primer can be used for interior and exterior surfaces, such as woodwork, plaster, and hardboard. Apply your water-borne primer with a brush, roller, or spray. We recommend a brush as this will help the primer to penetrate the surface and plug any holes. Clean your paintbrush afterwards with water.

Like water-borne primer, solvent-borne primer can be used on interior and exterior surfaces, including woodwork, cement, and building boards. This primer can be applied with a brush, roller, or spray. As solvent-borne primer is oil-based, you will need to clean your equipment with white spirit or brush cleaner.

White and pink wood primer is a general-purpose primer. It is non-toxic and hardwearing, meaning it can be used inside and outside. It is safe to use on surfaces which children and pets may come into contact with. Apply this primer with a brush and clean it with white spirit afterwards.

Aluminium wood primer is used on softwoods and on surfaces that have been previously treated with wood preservative. Apply with a brush and clean your equipment with white spirit.

Universal wood/metal primers can be used on new wood, masonry, metal, and plaster, inside and outside. Apply with a brush and clean with white spirit afterwards.

Primers for Plaster

It’s imperative to use a primer on bare plaster. This image shows what happens if you paint straight onto bare plaster without priming with a mist coat first.

Newly plastered walls and surfaces are porous, so using a primer before applying subsequent layers of undercoat and paint is key to a great finish.

Adding water to an emulsion paint gives you a mist coat that can be used as a primer on newly plastered surfaces. Follow the manufacturer’s guidance on thinning your paint with water. Mix together in a bucket and apply to your surface with a brush.

Stabilising solution is used on powdery surfaces. This primer creates a seal over the surface, stopping the powdery surface from interfering with your paint system, as well as preventing your paint from penetrating the porous plaster surface.

If you are painting building materials such as plaster, brickwork, and concrete blockwork, you’ll need an alkali-resisting primer. As the name suggests, this primer is designed to be used on surfaces that are alkaline in nature. Apply your primer with a brush, roller, or spray.

Spirit-based primer can be used on indoor and outdoor surfaces. The shellac-based primer is great for sealing over stains and odour on interior walls and ceilings, and can be used for spot-priming exterior surfaces. Apply with a brush, roller, or spray.

Metal Specific Primers

Painting the metal surfaces in your home and garden – think garden gates, garage doors, and aluminium window frames – is a good way to refresh your space. Before you start painting your metal surfaces, you will need to prime them to help the paint adhere to the smooth metal surface.

Primer comes in two basic types: single pack primer, which is one tin of pre-mixed primer solution, and two-pack primer, which comes in two tins that need mixing together before applying to your metal surface. Both can be applied with a brush, roller, or spray directly onto your cleaned metal surface and will help prevent the metal from corroding.

Etch primer is used on untreated ferrous metals including aluminium, zinc, copper, brass, tin, and steel. Apply with a brush, or spray.

If you’re painting a surface that is prone to rusting, zinc phosphate is a rust-inhibitive primer and will stop the metal from rusting. This primer is suitable for all ferrous metal surfaces and can be applied by brush or roller. You will need to clean your brush and equipment with white spirit.

Water-borne primer is a quick-drying primer that can be used on all metals. This primer is suitable for indoor and outdoor use and can be applied with a brush or roller.

Universal primer can be used on bare wood, metal, plaster, and masonry, so is a great option if you’re painting multiple surfaces around the house. Apply this primer with a brush or roller.

If you are painting new or galvanised metal, mordant solution is a primer which changes the surface of the metal, meaning your paint system will adhere to the surface. When applied, the solution will turn the metal black. Apply this primer with a brush.

Do You Need To Prime Before Painting?

In most cases, you should prime your surface before you start painting. For wood or metal painting projects, it is essential to prime your surface. Because wood is porous, a layer of primer paint stops your paint from soaking into the wood. Primer also helps your subsequent coats of paint to adhere to your surface – especially important for metal or glossy surfaces.

Sometimes if you’re painting interior walls, it is not necessary to use a primer. We’ve explained in more detail below when you should use a primer paint on interior walls.

Should You Use Primer Paint for Interior Walls?

In most cases, you probably won’t need a primer paint for your interior walls. There are some great high-quality paints on the market which render primer paint for interior walls somewhat redundant. Having said this, there are a few cases where you would want to prime your interior walls for a smooth finish.

If you’re painting over a dark colour with a lighter colour, priming the wall will help to cover the dark wall, and your new colour look more saturated and pigmented, without the need for lots and lots of layers. When painting over a dark coloured wall with primer paint, take as much care as you would with your new colour. If the primer looks patchy and uneven, your new colour will too.

Using a primer paint is a good idea if your surface is porous. Walls can become porous when moisture in the air is absorbed. Walls in kitchens and bathrooms are more likely to become porous than other rooms because of the increased humidity and moisture that builds in the air. Priming these walls before painting stops the paint being absorbed into the wall.

If your walls are stained, you will need to use a primer paint to cover the stain and to stop it affecting the new paint. Spills and leaks happen and whilst it can be frustrating, it doesn’t have to mean the end of a professional-quality paint finish. To ensure an even result, we recommend applying primer paint to the whole wall, not just the stain. This also applies for ceiling stains too.

If you’re painting over a glossy surface, it’s hard for the paint to adhere without the use of primer. If your walls have previously been covered with high-gloss or enamel paint, you will almost certainly need to apply a primer paint before you start painting and may even need to gently sand the surface before priming if it is very high-gloss.

You might find your walls have an odour and whilst unpleasant, walls can easily soak up strong smells like cigarette smoke or cooking smells. Because primer acts as a sealant, applying a coat will seal in the odours, neutralising the smell in the room.

Mist Coat vs Primer

A mist coat is a water-borne primer. You can make up a mist coat by adding water to an emulsion paint (follow manufacturer’s guidance on mixing your paint). The difference between a mist coat and primer is that a mist coat should only be used on new plaster. For priming other surfaces such as wood and metal, a wood or metal specific primer should be used.

What Is the Difference Between Primer and Undercoat?

Primer paint and undercoat may seem similar and, whilst there are some crossovers, they serve different purposes.

Primer is applied first and helps undercoat and subsequent layers of paint adhere to the surface you are painting. Like undercoat, a primer will help to smooth porous surfaces, but we wouldn’t recommend skipping your undercoat layer.

Undercoat is applied after primer paint and provides a smooth, even layer to apply your final coat of coloured paint onto. Some good quality paints combine an undercoat with the paint, resulting in a smooth and even finish without the need for a separate undercoat.

Woodwork Defects and When You’ll Need To Use A Wood Primer

If you find a defect in your existing woodwork, a wood primer can help to fix the problem.

Cracks in wood are common and can be caused by dry conditions or heat. If you find cracks in your wood, rub down the area with a fine abrasive in the direction of the grain, then use a primer on the area (see ‘Different Types of Wood Primer’ to find the best primer for your job.) Depending on the size of the crack, you may need to insert a stopper for a smooth surface.

Open joints are common when wood butted together during construction dries out and shrinks away. Fill the gaps with a wood filler and sand down with a fine abrasive, then use a primer over the surface.

Softwood can be especially prone to resin exudation, where the natural resin comes to the surface, staining the wood. This resin makes it difficult for paint to adhere to the wood. Apply an aluminium primer to your wood if this happens. The primer will stop the resin from seeping through and compromising your paint system.

End grain is the result of wood having been cut across the grain and is very absorbent. If you are working with wood with end grain, you’ll need two coats of primer to properly seal the surface and stop your paint and other moisture from being absorbed into the surface.

How to Apply Primer Paint

Although most primers can be applied with a brush, roller, or spray, it is best applied with a brush. This is because you have more control over the force used and can feel for any dry or thick areas on the surface better than when using a roller or spray. It is important to cover the whole area and to encourage the primer to penetrate the surface with the brush, especially porous surfaces such as wood and plaster.

How Many Coats of Primer On Wood?

If your wood is pre-treated, one coat of primer will be enough to stop your paint from being absorbed and to encourage the paint to adhere. If your wood is untreated, or if the surface has end grain, you will need two coats of primer to ensure an effective seal.

How Many Coats of Primer On Metal?

On most metal surfaces, one coat of primer is enough. Because metal is not a porous surface like plaster and wood, the primer doesn’t have to absorb into the metal. Its job is to ensure that subsequent layers of paint adhere to the surface and one coat will be enough to do this.

How Many Coats of Primer On Plaster?

On new plaster, we recommend applying two coats of primer. Raw plaster is very porous and to properly seal the surface, two coats of primer applied with a brush or roller is ideal.