What Does Exterior Paint Contain That Makes It Different From Interior?

Paint Composition

• Latex paints have four main components: Solvents, pigments, resins and additives. The composition of these parts varies depending on the quality of the paint and the surface for which it is intended. Both interior and exterior latex paints contain water as the solvent, which is what makes your paint wet. Solvents evaporate as the paint dries, leaving behind the resins, pigments and additives. Exterior paints contain chemicals to protect the painted surface from fungus and insects. These may not be safe for indoor use. Interior paint may not withstand heat and rain as well as exterior paint because it has no weather resistance.

Resins

• Resins enable the paint to adhere to the service and help to prevent blistering and peeling. The type of resin used will determine whether or not the paint can withstand scrubbing or whether it will fade under direct sunlight. Resins are also critical to ease of application, because the paint’s flow and leveling ability depends on the resin used. Both interior and exterior latex paints often have acrylic resins because these dry to a flexible, weather resistant and heat resistant surface. Interior floor paints may contain silicone or epoxy resins because, while floors aren’t subject to extreme temperatures, they must be able to withstand a lot of abuse -- such as scrubbing and scuffing -- so a rigid coating is needed.

Silicates

• The primary difference between interior and exterior latex paints is the additives given to enhance their durability. Also called kaolin or china clay, aluminum silicates are used primarily found in interior paints because they improve stain and abrasion resistance. Zinc oxide is a silicate used in exterior paints to provide stain blocking, aid in mildew resistance and to prevent corrosion.

Biocides

• Quality exterior paints contain mildewcides, fungicides and pesticides to protect the surface they’re covering. Mildewcides are added to exterior paint to prevent mildew growth on the finished surface. While not usually added to interior paints, mildewcides may be found in those formulated for use in kitchens and bathrooms, where moisture may be an issue. A preservative biocide is used to prevent bacteria growth in both interior and exterior paints. Paint may be stored for long periods, and the container may be opened and closed several times. Preservatives help prevent bacterial contamination from ruining the paint.