A friend decided recently that they were going to do some remodeling, and in the process, they wanted to repaint the interior of their house. She had already gotten started but wasn’t sure if she was using the right paint. When she asked me the difference between exterior and interior paint, I realized it was a topic that might help many people.
Exterior paint is made to survive inclement weather while resisting mold, mildew, fading, cracking, chipping, and more. On the other hand, interior paint is made to resist stains and be scrubbed clean without being damaged. Interior paint is also made with chemicals that are safer for breathing.
So, can you use them interchangeably? Will using exterior paint inside give you a more durable finish, or is it too much of a hazard? Let’s take a more in-depth look at the differences between interior and exterior paint and see if we can answer all of your questions.
Basic Ingredients of Paint
Before we dive into the differences between exterior and interior paints, it’s important to have a base understanding of how paint works and how it’s made.
There are only a few ingredients in paint, and each one is crucial. The main base of the paint, also called a solvent, is either oil or water. Oil-based paints tend to be used for exterior paints and are not generally found in interior paints. Water-based paints can be used for either interior or exterior paints.
Pigments give your paint color. Special additives can give your paint different physical effects such as water-resistance, UV-resistance, or make it more durable.
When you apply paint to a surface, the solvent or base will quickly start to evaporate. Once it does, you’re left with just the pigments and additives bounded to the surface you just painted by the resins.
Epoxy, silicone, and acrylic can all be used as resins. Since exterior paint needs to stick through more turbulent conditions, it usually uses harsher resins with superior adhesion.
Once the paint dries and the resins bond your pigments and additives to the surface, your wall is painted and looks brand new.
Exterior Paint: A Quick Overview
Meant for painting the outside of different buildings, exterior paint has additives to make it weatherproof. It can withstand rain, snow, sleet, and any other inclement weather. Plus, it’s built to handle years of hard sunlight without fading or cracking. It’s also very durable to avoid chipping and flaking when the wind blows twigs and other debris against your home.
To make a more durable paint, extra resins and additional additives are used. These will cause outgassing while the paint dries. Most of the off gassing will take place in a few days, but it will continue slowly for up to several years. Mildecides are also used to prevent mold and mildew. These can cause respiratory issues if inhaled.
Wondering if you can use exterior paint inside? Check out my article Can You Use Exterior Paint Inside? for a more in-depth look.
Interior Paint: A Quick Overview
Interior paint is made to cover the inside of your home. It’s not as durable as exterior paint, but it can withstand scrubbing and washing, though not on a constant basis. It’s not waterproof, though, and It won’t withstand the weather.
Since it’s meant for use indoors, interior paint is low in VOCs and other harmful chemicals. Outgassing is minimal to none. To avoid harsh chemicals, interior paints use organic pigments for color. In direct sunlight, those organic pigments will fade very quickly. They’re also more susceptible to cracking and flaking.
So, can you use interior paint outside? I’ve got a complete article on this topic: Can You Use Interior Paint Outside?.
What is the Difference Between Interior and Exterior Paint?
Now that we’ve covered the basics, it’s time to examine the differences between these two main types of paint. Both of them will change the color of whatever surface you apply them on, but that’s really where the similarities end.
Type of Binding Resins
The binding resin will determine how well the paint will adhere to a surface. Exterior paints use acrylic as the binding resin, which has an extremely strong bond and will offer the most durability. But acrylic can have harsh odors, so interior paints stick to epoxy and silicone instead.
Pigments
As mentioned, pigments give your paint color. To avoid adding extra chemicals into the mix, interior paints use organic pigments. These have no harsh odors and are completely safe to breathe. But exterior paints need durability more than they need to keep out the harsh chemicals, so you’ll find non-organic pigments in exterior paints that can help exacerbate the intense odors and fumes, but they don’t fade as quickly.
Different Additives
Because interior paints don’t have to deal with as many potentially harmful situations, they don’t have many additives. But exterior paints need to handle all sorts of different weather, debris, temperatures, and more. To help with this, additional additives are used that can help the paint to resist fading, handle changing temperatures, and even prevent cracking. Mildecides are also added to exterior paints to help prevent mildew, mold, and algae from growing.
Weather-Resistance
Interior paints are not expected to deal with harsh or changing weather conditions. Because of this, they have low temperature-resistance, low water-resistance, and they fade quickly in sunlight. But exterior paints are made to withstand all of these, so they have superior weather-resistance.
Durability
Exterior paint is built to withstand years of torment from the weather. It’s also made tough enough to not chip off from contact, such as twigs being thrown against your house by the wind or bugs flying into the wall. As such, it’s much more durable than interior paint. But interior paint is pretty durable as well; able to withstand occasional scrubbing and cleaning.
Resistance to Physical Damage
If it takes a direct hit, your interior paint will likely chip off, exposing the old paint or bare wall beneath. Exterior paint can also be chipped, but it’s much harder because of how much stronger the resins bond the pigment to the surface.
Levels of VOCs
VOCs are volatile organic chemicals, and they release fumes that can be hazardous if you breathe them in. Luckily, interior paints are very low in VOCs, and often devoid of them entirely. But exterior paints are not. Since they’re not intended for use inside, exterior paints are often full of VOCs that will outgas into the open air outside. But if this happens inside, it could be a health risk.
Adhesion
Both interior and exterior paints offer good adhesion. However, exterior paints adhere well enough to withstand all sorts of inclement weather, a claim that interior paints cannot mimic. In general, exterior paints stick better and last longer.
Dry Time
Oil-based paints tend to dry slower than water-based paints. But, paints that are outside in the open air will dry faster than paints inside without constant airflow. Finally, heat will also make the paint dry faster. When you combine all of these factors, exterior paint usually dries quicker, particularly if it’s water-based.
Fade Resistance
One of the most constant forces that your paint must contend with the outside is sunlight. The sun will beat on it for 12 hours or more of most days each year. That’s a lot of abuse. Thankfully, additives in exterior paint are made to resist this fading and will even protect against UV rays. However, interior paint isn’t meant to withstand direct sunlight. The organic pigments in interior paint will fade very quickly, especially since there are no additional additives to prevent fading.
Temperature Resistance
Exterior paint is made to withstand a variety of temperatures. It has to survive winter’s cold and summer’s heat, plus every temperature between. But interior paint is only intended to be in a small window of temperatures from about 60 degrees Fahrenheit to about 90 degrees where most people keep their homes.
Moisture Resistance
Since rain is a rather common occurrence in most places, exterior paint must withstand rainstorms, thunderstorms, basic showers, and more. Interior paint only needs to handle the occasional washing. Naturally, this makes exterior paint far more water-resistant.
Mildew Prevention
Interior paints do not generally have any mildewcides added. But on the outside of your home, algae, mold, and mildew are all real threats. To deal with them, mildewcides are added to exterior paints. These aren’t great to breathe in, but they’re very effective at stopping mold, mildew, and algae in their tracks.
Scuffing and Scratches Resistance
Both interior and exterior paint offer pretty decent resistance to scratching and scuffing. But at the end of the day, the durability of exterior paint wins out. It can resist much harder scuffing with no consequence.
Outgassing
Since interior paint is meant to be used indoors, it’s generally devoid of harmful chemicals, harsh additives, and dangerous VOCs. But exterior paint isn’t subject to the same regulations. Because of this, those chemicals and additives find their way into many exterior paints. It can make them more durable and weather-resistant, but it also means that exterior paints outgas while they dry. The majority of the gas is let off after a few days, but it will continue to outgas for several years.
Cost
The cost difference between these two types of paint isn’t great, but it’s there. Exterior paint has more additives, more chemicals, and holds up better, so it’s generally the more expensive option. Interior paint tends to be a bit more affordable, though it doesn’t offer the same durability and protection.
Interior vs Exterior Paint: Side-by-Side Comparison Chart
| Interior Paint | Exterior Paint | |
| Binding Resins | Epoxy, Silicone | Acrylic |
| Durability | Moderate | Advanced |
| Resistance to Physical Damage | Some resistance | Very resistant |
| Levels of VOCs | Low to no VOCs | Moderate |
| Adhesion | Good | Best |
| Fade Resistance | None | Resists sun fading |
| Temperature Resistance | Low | High |
| Moisture Resistance | Low | High |
| Mildew Prevention | No | Yes |
| Anti-fade UV protection | No | Yes |
| Scuffing and Scratches Resistance | Yes | Yes |
| Outgassing | No | Yes |
| Cleanliness | Made to scrub clean | Made to spray clean |
| Weatherproofing | None | Weatherproof |
| Cost | Moderate | High |
Indoor/Outdoor Paint
Paint is divided into just two categories, but that can still become a hassle when using paint for different projects inside and outside of your home. Fear not! There is a great way to simplify the entire issue.
Manufacturers took it upon themselves to make your life easier by introducing interior/exterior paint. As the name implies, this paint is just as effective whether it’s applied inside or outside.
Indoor/outdoor paint offers improved durability, weather-resistance, temperature-resistance, and UV-resistance over standard interior paint. But it’s also got fewer harsh chemicals and additives than exterior paint, making it much safer for use indoors.
You won’t find many VOCs in interior/exterior paint. It doesn’t have the harsh odors and off-gassing of exterior paint, but it’s also not quite as durable. Still, it represents the best of both worlds and makes it much easier to pick paint and use it for several projects.
If this route sounds appealing to you, then I recommend the Rustoleum Indoor/Outdoor Latex Paint. It doesn’t have a strong odor and is completely safe for use indoors. But it’s also very durable and will withstand any inclement weather or years of constant sun abuse.
Conclusion
Though it may seem trivial upon the first inspection, where you apply each specific type of paint can have very real consequences. If you use exterior paint on inside surfaces, you might end up with pervasive chemical fumes filling your home. Apply interior paint to the outside of your home, and you’ll likely find it cracking, chipping, and fading away in a very short time.
It’s best to use the type of paint that’s intended for whatever surface you’re painting, whether that’s inside or outside. Of course, you can always opt for an indoor/outdoor paint instead and cease the confusion entirely.
If you found the information contained in this article to be useful, please feel free to share it with others who might need the info as well. Also, I’ll try to respond to any questions or comments left in the comments box below as quickly as possible.