Interior Paint vs. Exterior Paint: What’s the Difference?

women chooses house paint at hardware store

by Rocky Vuong - September 22, 2021

One of the most commonly-asked questions we hear is: what paint should I use?

Most people start by selecting the paint colour and finish first, without even a second thought as to what’s actually in the paint. Understanding the basic chemical components of the paint you use is essential, as it can affect the finish, durability and even the health of you and your family.

Whether you’re painting the front of your house, or a feature wall in your bedroom, our comprehensive guide will help you to know the difference between interior and exterior paints (or indoor and outdoor paints), so you use the right paint for your project. 

Types of Paint

All types of paint fall into two broad categories: water-based paints (also latex or acrylic paint) or oil-based paints. Until about a decade ago, oil-based paint was widely used on exterior surfaces because it was more durable and affordable. However, it has been proven that it is more harmful to the environment and your health, which is why it’s been restricted in its use, except in small amounts in rare situations.

Water-based paint is now the common choice for use in both interior and exterior paint projects. There have been many improvements in the ingredients to improve adhesion and coverage and it boasts many advantages, such as a shorter drying time, minimal odour and it releases significantly fewer VOCs (Volatile organic compounds) during the drying process.

Water-based painted surfaces are also easier to clean and dry faster than oil-based paint. They resist yellowing and fading over time, if protected from sunlight, and are not flammable.

Main ingredients in all paints

different coloured paint cans

Both interior and exterior paints contain the same four basic components: Pigments, solvents, additives, and resins. Each ingredient plays a different role to produce the best 

Therefore, understanding what each component does helps to determine what type of paint to use for your next painting project:

  1. Pigments are finely ground particles applied to paint that make up the paint’s colour. There are two types of pigments: prime and extender. Colourants are prime pigments that provide the actual color in the paint can, while organic colourants provide bright colors. Inorganic colorants are earthy colors and are more durable for exterior applications.
  2. Solvents are the liquids that suspend the ingredients so that you can paint on surfaces. In water-based paints, the liquid is primarily water, whereas for oil-based paints, the liquid is paint thinner made from a variety of solvents, which could include acetone, turpentine, naphtha, mineral spirits, and more.
  3. Additives give paint different properties, which include thickening agents, leveling agents, surfactants, emulsifiers, and biocides. These help with colour accuracy, improve application, better adhesion and prevent mildew growth.
  4. Resins bind the pigments together to create the paint film. Oil-based resins are made from oil derivatives such as linseed oil. Resins are the key ingredient that directly relates to paints performance characteristics.

What’s the difference between interior and exterior paint?

The main differences between interior and exterior paints are resins and additives contained in them. The binding resins for interior paints are rigid – which allow interior paint to be cleaned easily – and are stain resistant. Vinyl acrylic resins are often used in interior paints to further enhance its scrub resistance and washability.

For exterior paints, the resins used are softer, providing for characteristics that help fight against fading, mildew, temperature changes and moisture. Exterior paint must also be tougher to resist peeling and chipping, which is why 100% acrylic resins are used for best results. They are regarded as the best overall. The benefits of acrylic resins include better adhesion and alkali resistance, which means the paint is less likely to burn over fresh concrete or masonry.

Paints also contain additives to regulate drying time, improve weather resistance, and keep the pigment distributed evenly in the paint solution. Interior paint’s lack of certain environment-specific additives makes it unsuitable for use on exterior surfaces.

Meanwhile, exterior paints are designed to withstand extreme weather conditions. So, the strength of some additives used in these paints may be unhealthy in enclosed indoor areas. Interior paint wouldn’t last long if you used it to paint the outside of your house, because it lacks heavy-duty additives – which is why painters never recommend using interior paint to paint the outside of your home.

Not sure which paint to use?

If you’re a painting novice and are still not sure where to start, then you can check out our tips to choose the right paint. Alternatively, it’s always worth asking a staff member at your local hardware store or Bunnings, or consulting a professional painter for some expert help. Newline Painting will help you to decide which paint is best suited to your painting project for the best results and to make sure it lasts for years to come. Get a free quote for our painting service today.

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