All paints contain pigments, resins and additives. Resins bind the paint to the surface and can be made of acrylic, epoxy or other materials.

Pigments give paint its color and can be organic or inorganic. Additives provide specific properties like handling mildew or having a faster drying time. Solvents are the liquid that suspends these ingredients, typically water for a water-based paint or mineral spirits for oil-based paints.

Resin

Resin is a dangerous substance to work with and should be used with proper safety precautions, especially when painting. This includes wearing gloves, goggles and a respirator. It also requires a well-ventilated area. It is important to prepare a surface thoroughly for painting with resin for optimum adhesion and a smooth finish.

The resin in exterior and interior paint is made to withstand a wide range of temperatures, from winter cold to summer heat and every temperature in between. This is a big reason why exterior paint is much tougher than interior.

It also must withstand rainstorms, thunder showers and basic washing, which is why it is water-resistant. In addition, it must resist fading from the sun’s UV rays. This is why additives are included in exterior paint to prevent fading. In contrast, interior paints do not have additives to protect against fading.

Pigment

All paints contain a combination of pigments, additives, resins and solvents. But it's the pigment that gives paint its color and binds it to the surface. Paints that contain too much of one pigment may be muddy and difficult to clean or dry.

The pigment also determines the fastness of a color against light, gases, acid or alkali. This is known as its "lightfastness" or "light resistance."

The resins used in exterior and interior paint differ because they need to handle different weather conditions. Resins in interior paints are typically more rigid to resist cracking, flaking or peeling from temperature changes. They also help the paint hold onto rough surfaces like textured walls.

Solvent

As the name implies, this is the liquid component of paint. It’s also what binds pigments to surfaces and is what evaporates during the drying process. It can contain a variety of ingredients, including volatile organic compounds (VOCs). VOCs aren’t good for you and can cause a wide range of short- and long-term health problems, such as headaches, dizziness, respiratory issues, liver damage and certain cancers.

Exterior paint has to deal with the sun beating down on it for 12 hours of most days, so additives in this type of paint are designed to help the pigments resist fading. Additionally, exterior paint must deal with temperature changes and other environmental debris, so binding resins are made to let the coverage expand and contract without cracking.

Interior paint doesn’t have to deal with such a variety of environmental factors, so it contains less additives. However, mildew and mold can be a problem in rooms inside your house, so mildewcides are added to this type of paint to keep these organisms at bay.

Additives

Additives are what give paint its final physical qualities. These include things like mildew resistance, surface adhesion, moisture control and gloss retention.

Binders are an important additive that helps bind the pigments together to form a dry film. The type of binders used in interior paints are different from the ones used in exterior paints, providing characteristics that are needed for indoor use, such as scrubbability and staining resistance.

The binders in exterior paints must also be able to resist harsh conditions like moisture, temperature changes and fading from sunlight. They do this by incorporating ingredients such as mildewcides, anti-fungal agents and corrosion inhibitors.

While it may be nice to paint a room with zero odor, it’s not realistic with exterior paint. This is because as the paint dries and cures, it will off-gas or release volatile organic compounds (VOC) into the air. These VOCs are harmful to both the environment and your health. To minimize this, all latex acrylic exterior paints contain an ingredient known as an odor counteractant.