Paint, primer and undercoat buying guide

An undercoat, as its name suggests, is the first layer of paint applied to a clean, dry substrate, before the finishing coat. Although it might be tempting to cut out this step - lazy as we are! - it really does make a big difference!

In fact, an undercoat offers many advantages - such as improved adhesion, ease of application and removal of subsequent paint (in case you have a change of heart) and durability of the end result. It also acts to stabilize porous, flaky, fragmented or stained surfaces. In the jargon, this is known as "blocking in the base". This prevents differences in absorption between areas of the substrate - such as plaster - which can otherwise produce unattractive stains.It also allows you to save money, as the substrate will suck up less paint.An undercoat also lets you standardize the substrate (if it has a different existing paint colour), apply a water-based paint over old oil-based paint or avoid the appearance of rust  (iron) or tannins (wood) which can otherwise produce marks or stains.

All things considered, it's well worth applying an undercoat before you crack out the house paint!

Just like finishing paints, primers fall into three categories.

Vinyl

Vinylics are water-based paints low in pigment, useful for priming plaster or plasterboard to make it less fragmentary and porous.

Acrylic

Just like vinyls, acrylics are water-based, but they're also high performance paints. They're non-flammable and come in three varieties of finish: matt, satin and gloss, suitable for both priming and finishing of walls, ceilings and woodwork.Both environmentally friendly, vinyls and acrylics dry quickly, are practically odourless and very easy to use (brushes and rollers can be cleaned with water).

Glycerophthalic acid

The solvent here is white spirit or turpentine ("turps"). Made up of synthetic resins, glycerophthalic paints can be applied directly over alltypes of paint, bare wood or wallpaper. They are very resistant and give a good render and a nice finish on woodwork in particular. Used as a primer or undercoat, their high opacifying power and even consistency produce an excellent finish, especially on stained or damp walls. Only drawbacks: long drying time, strong smell during application and necessity of cleaning rollers and brushes with white spirit.

As well as classifying these paints by chemical composition, they can also be divided into universal, plasterboard and cover-all types.

Universal undercoat

This type of paint is multi-purpose and renders the substrate uniform, concealing imperfections, and improves adhesion of the finishing paint. It also makes application and removal of other wall products easier, can be applied in any room (dry or wet) and can be covered over with any type of top coat (glycero or acrylic). It is best suited to interior surfacesin goodcondition. It provides strong adhesion on plaster, plasterboard, wood and cement, and gives economical coverage in large volumes.

Plasterboard primer

Microporous (allowing air to pass through so the substrate can breathe), it adheres well to substrates such as plaster tiles, plasterboard, plaster, wood, cloth and fibreglass. This underlay facilitates application and removal of other wall products, masks edges of plates / boards and blocks the porous bottom layer of plaster to make it less fragmentary and absorbent. Good for use on new porous substrates since it has greater opacity (i.e. covering power) than universal undercoat. Specialized "plaster or cement" primers are good for hiding stains or blemishes to ensure a homogeneous finish.

Cover-all undercoat

With adhesive and insulating properties, this type of primer neutralizes stains (nicotine, tannin, damp) by blocking from rising to the surface. Multi-purpose (smooth surfaces, wood, radiators, plaster, wall tiles etc.), it can be covered with either acrylic or glycerophthalic top coats. Finally, it also produces a strong bond on any conventional substrate, whether indoors oroutdoors.

Although most "universal" primers are intended for certain types of materials (such as tiles or plastics), more unusual materials such as PVC, laminate, cast iron or iron may need a specialized underlay to ensure good adhesion of subsequent coats of paint.

Primers for wall tile / decorative plaster

Ideal for waterproofing the walls of your kitchen, bathroom or shower cubicle before laying tiles over the plasterboard, it also improves theadhesion of heavy decorative cladding such as Wood, waxed concrete, render, heavy vinyl wallpapers etc.

Primers for tile, glass, laminate

Easy to paint over with any type of paint (glycero or acrylic), these primers have a high opacifying power and increase the adhesion of finishing paint on very smooth surfaces, such as tile, glass or laminate cladding.

Primers for interior wood

These microporous paints let the substrate breathe and improve the adhesion of finishing paint. Designed especially for woodwork (doors, window frames, skirting boards, etc.), they're suitable for indoor use and in any room (dry or wet).

Primers for plastic and PVC cladding

These reinforce top coat adhesion on plasticized or PVC surfaces, and are resistant enough for exterior use. Can be covered over with any type of paint (e.g. glycero or acrylic).

Wet surface primers

Solidifying and insulating, these paints regulate water absorption in your substrate. Can be applied to new and difficult substrates, porous or "wet" (bathroom, kitchen) surfaces or in rooms subject to increased humidity (a room with a tumble dryer, for example). They stabilize walls before subsequent paint is applied, providing resistance against mould and condensation. These microporous paints take well on plaster, plasterboard, wood and fibreglass.

Primers for ferrous metal

Easy to paint over, these provide effective protection against rust in ferrous metals and reinforce the adhesion of finishing paint. Can be covered over with any type of decorative paint (make sure you use an appropriate weatherproof paint if outdoors).

Aluminium, copper, zinc and galvanized metal primers

These primers are resistant to corrosive chemicals and ensure good bonding of top coats applied to galvanized, aluminium, zinc and copper substrates. Can be covered over with any type of finishing paint (e.g. glycero or acrylic).

As you've now seen, there are many varieties of undercoat (or primer), with prices varying from single well into double figures for the same quantity of product! These are the key criteria that differentiate the many varieties of primer.

Opacifying power

This is the ability of the paint to create a thick covering capable of masking the base or existing colour of the substrate. Simply put, the greater the opacifying power of a paint, the more uniform the finish.Low-end undercoats are often much more heavily diluted and therefore have fairly limited opacifying power.

Filling power

A measure of the paint's ability to smooth out surface irregularities. Of course, the effect will be limited - it's essential to prepare your walls (i.e. plaster etc.) before applying undercoat (only to be used on clean, dry, solid surfaces).

Coverage

Represents the surface area that can be covered per litre of product. This is important as it lets you calculate how much paint you'll need to buy. Just like with finishing paints, the greater the coverage, the less paint you need - so the more money you can save.

Tips for choosing

Some paints contain chemical agents equivalent to an undercoat so you an apply them straight off in one go! If it's just a small job you've got to do, consider the available colours (some brands have a fairly limited range, such as: gray, red, green, beige, yellow, black, white).

If you have a much larger surface to paint (tile, furniture, etc.), it's often more economical to apply an appropriate undercoat and then cover it with a conventional top coat - giving you a much wider choice of colour, texture and composition (glycero or acrylic).Avoid breathing in those nasty VOCs (volatile organic compounds) by equipping yourself with a protective mask!

Brushes at the ready...

To find out more about home decoration, follow our editors' advice and check out their other guides:

How to choose your plaster?How to choose your wall and ceiling paint?How to choose your decorative paint?How to choose your respiratory protective gear?How to choose your abrasives and sandpaper?How to choose your flooring?How to choose your interior floor tiles?How to choose your wall and ceiling paint?How to choose your paintbrushes?How to choose your paint rollers?How to choose your painting tools?How to choose your masonry tools?