Have you just finished painting the outside trim or walls and are wondering, can you use exterior paint inside? Or maybe you’re thinking it would be better to use in the bathroom with all the humidity? Although it may sound like a good idea, it’s exterior paint for a reason.
Exterior paint isn’t recommended for use indoors. It is a more potent blend of VOCs (Volatile Organic Compounds), pigments, additives, and binders. The finish is softer, more flexible, and produces toxic fumes even after it’s dry. It can make people and pets ill, and the finish isn’t as desirable.
In this article, we’ll examine what exterior paint is, whether it can be used indoors and if it is safe to use on interior surfaces. We’ll also discuss mixing interior and exterior paint. When you’re finished reading you’ll have a better understanding of exterior paint and if it’s wise to use indoors.
What Is Exterior Paint?
Exterior paints are designed to withstand blistering sun, sandblasting winds, torrential downpours, and earth shattering cold. When applied to buildings, structures, and outdoor surfaces, it must withstand the elements and not fade, peel, crack, or allow mold and mildew to grow.
Better made exterior paint contains high-quality flexible resin binders that expand and contract with the surface they cover. There is a blend of additives for leveling and to protect against staining, fading, and mildew. Most exterior paints also include UV protection to help the pigments maintain their color. Exterior paints contain more resins and tend to be softer than interior paints so they can adjust to temperature and environmental extremes.
There are oil and water-based paints for outdoor use. The base is the solvent that evaporates, and the binders hold the pigment and additives together to form the paint film. Paints are available in a range of finishes and additives and can be applied with a brush, roller, sprayer, and sponge. Water evaporates more quickly than oil, so water-based paints commonly dry more quickly. Expect to pay more for quality paint that will last 10 to 15 years when applied as directed.
Bases
Exterior paints were commonly oil-based but modern formulations have made some water-based paints as good as or better than the oil paints. Tung, linseed, or soybean oil, or synthetic alkyds make up the oil-base, while water is the base for latex or water-based paints. Oil-based paints tend to give off more VOCs than water-based, but the stronger chemicals are present in both. Both base styles are available in flat, satin, semi-gloss, gloss, and enamel finishes.
Pigments
Pigments are organic or inorganic and give the paint its color. White is inorganic in composition and commonly titanium dioxide. Organic is from plants or animals and inorganic is mineral in origin. Organic pigments produce a more transparent color and inorganic is more solid. The higher the pigment particle count, the richer and more solid the color. Exterior paint pigments are formulated to prevent fading due to UV, sun, and weathering.
Additives
Pigment, additives, and binders are carried and spread by mineral spirits in oil paint or water in water-based paints. The spirits or water evaporates, leaving the dry paint film behind. Additives refer to anything added to the paint to make it thicken, fill holes, level, stick to different surfaces, be more durable, harden, and resist UV and mildew. It also includes texture and reflective additives, and even scents to mask paint chemical smells. Exterior paints contain different additives than interior paint which allows it to withstand harsh environmental conditions.
Binders
Binders are what hold the pigments and additives together to form the paint film on different surfaces. They too are formulated to withstand harsher exterior conditions than those used on interior walls.
Can You Use Exterior Paint Inside?
You may have some leftover exterior paint or be of the mindset that since exterior paint withstands the elements so well, it should be excellent for interior use. While your logic makes sense, it doesn’t take into account the fact that interior and exterior paints are developed for different conditions. Although both types of paint may contain mildewcides, fungicides, UV blockers, and other common additives, they aren’t the same ingredients.
Exterior paints contain additives and binders designed to withstand harsher conditions. They are also formulated from materials that can be harmful in enclosed areas but are less so outdoors. Exterior paints commonly produce more VOCs and offensive odors that aren’t pleasant in confined spaces. The composition of materials in exterior paint commonly also makes it more expensive than comparable quality interior paints.
Using exterior paints in well-ventilated areas is an expectation when applying it. The paint will off-gas VOCs until fully cured; several weeks in warm weather to months when colder. Using it inside cabanas, sheds, or detached garages and other outbuildings is acceptable if there is good ventilation and it isn’t used for sleeping.
Although you can use exterior paint indoors, it isn’t recommended. Interior paints are designed for interior walls and ceilings like drywall. They dry much quicker and produce a harder surface to withstand knocks, scuffs, and abrasive cleaning. Exterior paints are designed for surfaces other than drywall and they expand and contract with thermal and mechanical expansion and contraction of the elements. Additionally, exterior paint commonly has a different texture or feel when dry that may not be as pleasing indoors as out.
Is It Dangerous to Use Exterior Paint Indoors?
Using exterior paint on interior walls is not recommended for many reasons, including cost and finish. However, the main reason is related to your health as the painter, and the health of those occupying the interior space. Exterior paints contain harmful VOCs (Volatile Organic Compounds) and chemical smells that can cause respiratory, neurological, and other issues.
Paint designed for outside use needs to be more flexible and therefore ‘softer’ or rubbery than interior paints. They take longer to cure than interior paints and continue to produce odors long after the surface feels dry. Since the odors are outside, they don’t normally pose a health concern once applied. Using them in enclosed spaces though is where they become much more dangerous.
Exterior paint contains mildewcides and fungicides made of harsher chemicals than those used in interior paints. Mildewcides and fungicides in interior paint are not the same. Additionally, the binders and pigments are made with different ingredients that withstand weathering better. Many professional painters wear respirators when applying exterior paint outdoors due to the fumes, especially when using a sprayer. Becoming lightheaded or dizzy standing on a ladder is a recipe for disaster.
Applying exterior paint on interior walls or ceilings releases the VOCs and chemical fumes where ventilation isn’t as good as outside. The VOCs are necessary to hold the pigment in the paint and bind it where it is applied. They are an even greater concern if applied with a sprayer instead of a brush or roller as the fumes are released into the air more quickly.
VOCs and chemical odors from exterior paint are dangerous to humans and pets. They can cause headaches, light-headedness, nausea, irritated eyes, and throat, respiratory issues, and concerns for those with weak immune systems. Prolonged exposure can also cause more serious illnesses and even cancer. Fumes can also be circulated throughout a home by central heating and cooling systems. The risks are less in well-ventilated spaces such as outdoors.
The odors from exterior paints can linger in interior spaces as they oxidize and cause health issues even after they have cured. I painted the interior walls of an uninsulated tool shed with exterior paint almost 5 years ago. I figured since the walls expand and contract with temperature extremes it would be a good solution. Even though it is well vented, there are days when I open the door and get hit with the fumes.
Can You Mix Exterior Paint With Interior Paint?
Dumping interior and exterior paint into one bucket or can is easy but it may be a waste of both paints. Mixing exterior paint with interior paint or vice-versa isn’t recommended. Even if the paints have the same base and manufacturer, the paints aren’t formulated the same. They have different binders, pigment, and additive compositions.
Exterior paint is blended to withstand the sun, wind, rain, snow, humidity, and other outdoor extremes. It has adhesion and elasticity requirements that interior blends don’t have. The chemicals and other ingredients used in exterior paints make it incompatible with interior paints.
Interior and exterior paint formulas have been constantly changing to better sustain specific conditions inside or out. What worked 5, 10, or 30 years ago may not be true today. Blending water-based with water-based or oil-based with oil-based of the same sheen may produce a usable paint, but where do you use it?
It will be a diluted exterior paint and may not withstand the extremes of the outdoors. Exterior blends contain more chemicals and VOCs that make it toxic to use indoors, even after it is dry. Additionally, the exterior paint has different levelers than interior paint, binders for surfaces other than drywall, and additives designed for harsher conditions. So, while it’s easy to pour leftover interior and exterior paints into one can, they won’t necessarily produce the desired results.
Painting surfaces with a blend of leftover interior and exterior paint often creates a film with a different texture, durability, drying time, and adhesion qualities. The finished results may bubble, flake, crack or peel. And even if the color hues are the same, to begin with, they may also change. So, while you can mix them, you shouldn’t.
Conclusion
You can use exterior paint indoors, but it may take weeks to months to fully cure. It releases fumes long after the surface seems dry and may make people and pets ill. The finish is flexible and softer, so not desirable for interior surfaces. If used on interior walls there should be exceptional ventilation and not where people or pets sleep. Additionally, exterior paint is more expensive.
I hope you better understand what exterior paint is and why it shouldn’t be used on interior surfaces. If you found this article interesting or helpful, please share it with others. Your comments and suggestions are always appreciated.