Paint, primer and undercoat buying guide
Anne, Painter, Cambridge26 guides
- Glycerophthalic acid
- Opacifying power
- Filling power
Why use an undercoat?
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!
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.
What different types of primer are there?
Just like finishing paints, primers fall into three categories.
Vinylics are water-based paints low in pigment, useful for priming plaster or plasterboard to make it less fragmentary and porous.
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).
The solvent here is white spirit or turpentine ("turps"). Made up of synthetic resins, glycerophthalic paints can be applied directly over all types 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.
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 surfaces in good condition. It provides strong adhesion on plaster, plasterboard, wood and cement, and gives economical coverage in large volumes.
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.
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 or outdoors.
What specialized primers can you get?
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 the adhesion 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).
How should I choose my undercoat?
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.
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.
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).
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
Avoid breathing in those nasty VOCs (volatile organic compounds) by equipping yourself with a protective mask!
Learn more about interior decoration...
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?
Anne, Painter, Cambridge 26 guides écrits
I’m also a self-taught DIY and decorating enthusiast. I love to find and restore furniture, creating one-of-a-kind decorative elements. Last summer I completed the restoration of my sister’s house with my niece. We did it all: electrical work, tiling, finishing, plasterboard. And today, whenever I get the chance to share my experience, I'm happy to do it. Here's a typical DIY joke: ‘What’s the difference between a clown and a DIY enthusiast? One has paint all over his face, the other makes people laugh.’ Happy tinkering, everyone!