The first thing you will notice when shopping for paint is that they are labeled interior or exterior. One is designed for use indoors and one for use outdoors. But is there really a difference between the types of paint? Do you really need exterior paint or could you just use some interior paint outdoors?
Technically you can use interior paint outside, but it is not advisable. Interior paint does not have the necessary additives to withstand rain, sunlight, temperature fluctuations, or mold and mildew growth. The bonding agents are also different. Using proper exterior paint will give you a better, long-lasting finish.
If you want to learn more about the differences between interior and exterior paint, and how to know what paint you should use, this guide has everything you need to know.
What is Interior Paint?
Interior paint is paint that is designed for use indoors. The emulsion that you use on the walls in your home, for example, is interior paint. So is paint that people use for their wooden furniture. Interior paint will stand up to some general wear and tear and it won’t rub off if you spill a little water on it.
However, it wouldn’t be able to hold up to sustained rainfall without washing off. Because they are used indoors, interior paints do not usually have harsh chemicals that would cause the paint to give off harmful fumes.
There are two main types of paint; oil-based and water-based. A lot of interior paints are water-based because oil-based paint tends to have more strong chemicals and VOC (volatile organic compounds) which are harmful to health when breathed in. However, this is not a hard and fast rule because there are oil-based interior paints too.
What is Exterior Paint?
Exterior paint is paint for outdoor projects. It could be the masonry paint that you use to paint the outside of your house, or paint for wooden furniture in your yard. Exterior paint needs to be a lot tougher than interior paint because it is exposed to the elements. It must be water-resistant so it doesn’t run off as soon as it rains.
It also needs to be able to withstand temperature fluctuations, frost, and UV damage without cracking. Naturally, the paint will get damaged eventually, but exterior paint is designed to hold up a lot better than interior paint.
Again, exterior paint can be water-based or oil-based. There are more oil-based exterior paints, especially for wood, because there are less concerns about fumes from strong chemicals when working outdoors. Oil-based products are also better at resisting water.
Both water-based and oil-based exterior paint needs special additives to make it water-resistant and to prevent UV damage.
Read Related: Can You Use Exterior Paint on Interior Walls?
What’s the Difference Between Interior and Exterior Paint?
The major difference between interior and exterior paint is where they are used. One is used indoors and one is used outdoors. Because of the different uses, the formulations are different too. While both can be oil-based or water-based, you tend to find that interior paints are more likely to be water-based and exterior paints are likely to be oil-based, but this is not a rule.
The main difference is that exterior paint will have specific additives in it to protect it against the elements. There is also a big difference in the type of bonding agent (resin) that is used.
Exterior paints use a soft, flexible resin to bond the paint to the surface. This allows it to expand and contract, and it is more resistant to chipping and fading in the sun. Interior paint, on the other hand, has a more rigid, brittle resin. This makes it easier to apply, but it is less durable once dry.
There is more off gassing with exterior paints too. This means that the paint gives off fumes. Most of this happens in the first few days after painting but will continue long into the future. That’s why exterior paints are not usually safe for use indoors because you will breathe in all of those fumes.
What are the Additives in Exterior Paint that Make them Better for Outdoors?
There are several different additives that are used in exterior paint. They include:
Fungicides and Insecticides
When paint is outside, it is exposed to a lot of moisture, which creates the perfect environment for fungus and mildew to grow. If this happens, the paint and the surface can be damaged, especially wooden surfaces. There are also potential issues with insects eating the paint and damaging the protective finish.
Exterior paints often have fungicides and insecticides in them to stop this. Common fungicides include chlorothalonil, octyl isothiazolone, zinc pyrithione, N-haloalkyl thiol compounds, and Diuron. These are not necessary in interior paints because there is less moisture and insects.
Water Repellent Additives
Water repellent qualities are crucial in exterior paint because it needs to be able to withstand the rain. There are a number of different additives that can make paint water-resistant, depending on the type of paint.
Water-based paints often use latex or ceramic additives to protect them. Epoxy is also very common as it has natural water-resistant properties. Oil already repels water, so oil-based paints don’t need as many additives to protect them against the rain.
Dirt Repellent Additives
Outdoor surfaces are exposed to a lot more dirt than indoor ones. If that dirt sticks to the outside of your home, for example, it can quickly make it look scruffy. So, some exterior paints have dirt repellent qualities to help them stay cleaner for longer. Usually, silicone-based paints are best at repelling dirt.
Being able to withstand temperature fluctuations is important for exterior paints. Exterior paint is often used on the outside of the house too, so having additional insulating properties is always a bonus. As such, some exterior paint may contain insulation agents, usually ceramic microspheres. Although these are not as common as other additives, they can be found in some paints.
What Will Happen to Interior Paint Outside?
Interior paint is not designed to be exposed to the elements, so you will encounter a lot of problems if you use interior products for outdoor projects. Interior paints are usually thinner than exterior paints, so you will notice a difference when you first apply it. It won’t adhere to the surface as well as you would expect, and the finish won’t be great. You will struggle to get an even coat and you will end up using a lot of paint to cover the surface.
Once it is dry (which may take a long time as it is not being used in the right environment), further problems will occur. When it rains heavily, the paint will start to degrade. It won’t wash off immediately, but it won’t last long. You will also notice cracking and peeling due to temperature changes and exposure to sunlight. The paint will also go chalky very quickly. All in all, the finish will look awful and after a short while, the paint will all start coming away.
What to Do if You Accidentally Used Interior Paint Outside?
If you accidentally use interior paint outside, you can’t just leave it. You have a few options here; firstly, you can paint over it with the right paint. Alternatively, you can wash the paint off and then start again, making sure to use the right products this time. In most cases, this is the best option.
If you have painted a surface like the outside of your house, you can use a pressure washer to quickly remove the paint, just remember that this will make a mess. You can also use some warm water and a sponge to get it off. Remember, it’s not designed to be exposed to water, so it should be fairly easy. The wall underneath is already prepped to paint, so simply wait until it is dry and start again.
Can You Apply Exterior Paint Over Interior Paint?
Yes, technically you can, but you should be very careful here. The most important thing is that you get good adhesion with the exterior paint, so the finish lasts a long time. But if you have already put interior paint on the surface, this can prevent adhesion.
If the interior paint did not bond to the surface particularly well and it’s already starting to peel, painting over the top of it won’t fix the problem. Unless the paint has bonded very well, it’s best to clean it off before applying your exterior paint because you will get a better finish that lasts longer.
Can You Mix Interior and Exterior Paint Together?
Technically, you can mix interior and exterior paints together, but you should seriously consider why you want to do it and what the benefits are because there are a lot of downsides. Paint is carefully formulated to get the right ratio of solvents, pigments, binders, and additives.
When you mix two different paints together, you can’t predict how all of those ratios will change and how the different ingredients will react. In other words, you’ve got no idea how the paint will go onto the surface and what the finish will look like when it’s dried. It’s common to get inconsistent colors or a patchy finish when you mix paints.
The VOCs will also be increased if you add exterior paint to interior paint. If you are applying the paint indoors, this could be potentially dangerous. On the other hand, if you are applying it outdoors, you have simply watered down the additives that protect the paint from the elements, so it won’t last as long.
In most cases, I would say you shouldn’t mix interior and exterior paint together. The only real reason that you would consider it is to get a specific color. The thing is, there are so many paint brands out there that if you just search around, you will be able to find the color you are looking for. All of the downsides to mixing interior and exterior paint mean that, 9 times out of 10, it’s not worth it.
Yes, there are some paints that are designed for interior and exterior use. Rust-Oleum sells a Universal All-Surface Spray Paint that can be used indoors and outdoors. They also have a brush grade version of this paint, which is available in the UK, but it is not available elsewhere at the moment. You often find all-purpose primers that can be used on any surface you like.
These products have the durability of an exterior paint but they don’t use lots of VOCs and harsh chemicals to achieve it, so they are safe for use indoors. These can be useful, in some scenarios but, most of the time, it’s better to use a specific interior or exterior paint because you’ll get better results.
Read Related: How to Seal Painted Wood for Outdoor Use
Can You Use Exterior Paint Inside?
No, it’s not a good idea to use exterior paint inside. The finish will be fine and the paint will last a long time because it’s very durable. The problem is the ingredients in exterior paint are not designed to be used in confined spaces. The strong chemicals in exterior paints can be harmful to your health and if you use it inside your home, you will constantly be breathing them in.
The fungicides and insecticides are particularly toxic. People wrongly assume that once the smell has gone, all of the harmful chemicals have gone, so you just need to ventilate the area for a few days. But that isn’t true and off-gassing continues for a long time. So, avoid using exterior paint indoors.
Paints are labeled as interior and exterior for a reason. The requirements of the paint are different depending on where it is used, and the ingredients are different as a result. Using interior paint outdoors will result in an uneven finish that doesn’t last.
Using exterior paint indoors can cause some serious damage to your health. So, put your trust in the manufacturers and only use the recommended paint for the job. If you do make a mistake, clean the surface and start again.
Jamie is the founder of The Backyard Pros. When he was 15 years old he started working at a garden centre helping people buy plants, gardening products, and lawn care products. He has real estate experience and he is a home owner. Jamie loves backyard projects, refinishing furniture, and enjoys sharing his knowledge online.