Different paints come with different properties, as they are manufactured for different purposes. Exterior paint is formulated to handle mildew and fading. Interior paint, on the other hand, is made to allow cleaning and resist staining.

To understand the difference between these two types of paint, one needs to gain knowledge of the chemistry behind each type. Let’s take a closer look at both types and see how exactly they differ from one another:


The Basics

All paints are formulated from the same basic ingredients. These include resins, additives, solvents and pigments.

The component that causes paint to be “wet” is the solvent. This component evaporates over time as the paint dries. What’s left is the rest of the components - additives, pigments and resins.

Out of these lasting components, the pigment is what gives the paint color. Additives, on the other hand, give it different properties, like mildew resistance. Finally, resins bind the paint to the surface. They’re typically made of silicone, acrylic or epoxy.

In both the exterior and interior paint, the pigments and solvents share similarities. For outdoors, water-based or oil-based paints can be used. For interior work, however, oil-based paints are not recommended. This is because of their odor and the fact that they’re difficult to clean.


What is the Difference Between Interior and Exterior Paint?

Even though there can be many subtle differences between these two types, the primary one is the choice of resin. As we already mentioned, the resin is the component that binds the pigment to the material you’re painting on.

Exterior paint has to survive being exposed to moisture and huge temperature changes. It also has to resist fading, chipping and peeling. Because of these reasons, the resins used in the manufacture of exterior paints have to be softer.

For interior paints, huge temperature changes and moisture are not an issue. Therefore, the resins used in their manufacture can be more rigid. This, in turn, cuts down on smearing and scuffing.


Characteristics of Exterior Paint

  • This type of paint is exposed to various weather conditions. Therefore, it has to provide protection against moisture caused by rain and snow, and ultraviolet radiation caused by sunlight. It also has to protect against fungal growth.
  • These paints are made to combat fading and mildew. Facing the UV radiation mentioned above, as well as very high temperatures, they need to be fade-resistant.
  • As already mentioned, the resins used for exterior paints are soft. This also makes them very flexible. As such, they don’t easily crack on contraction or expansion. This allows them to survive the ill effects of moisture and temperature changes.
  • By simply changing the sheen, this type of paint can be applied to various substrates. During rain, a house’s exterior absorbs small amounts of water. Flat paint won’t bubble - it will allow the water to escape.
  • As it releases more volatile organic compounds, exterior paint is unsafe for indoor use. Moreover, it requires sunlight to cure.


Characteristics of Interior Paint

  • This type of paint is used for decoration and aesthetics. At the same time, interior paint adds properties of dampness prevention, washability and easy maintenance.
  • Interior paint is formulated in such a way that it can withstand abrasion. As it occupies the same space as people, it is also designed to be more delicate than exterior paint.
  • This type of paint resists staining and can be scrubbed. As it doesn’t have to deal with sunlight, it has no fade-resisting properties. Also, it doesn’t need sunlight to get cured.
  • It is also washable. Roller and brush marks, as well as marks and spots left behind by kids and pets, can be easily cleaned.
  • Interior paint needs to have a low amount or zero volatile organic compounds. This reduces health risks and is vital to preserving air quality in one’s home.


Where Can One be Used Over the Other?

In exterior paints, the added resins can cause outgassing. This process typically doesn’t last longer than two days. However, outgassing can continue for years, albeit in much smaller amounts. This is why exterior paint should never be used indoors.

For stucco and masonry, a flat sheen exterior paint should be used. This allows surfaces such as these to breathe, letting the moisture escape through the paint. This is particularly important for brick walls.

As mentioned, interior paints are much more delicate. Moreover, they do not outgas in the same way, so using them indoors is safe. Adequate ventilation during painting is still important, though.

As a final note, there are also paints that can be used both outdoors and indoors. These “universal” paints are extremely versatile but do come with their own disadvantages. If you’re in doubt about which type of paint you should use, talk to a professional painter.

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