When you are deciding what paint to buy for your home, there are a lot of things to consider, and understandably, most of those are going to be about how your home is going to look. What colour combinations are you going to use? Will you use matte finish, or gloss? Will you use paint on your wooden surfaces, or solid colour stain? (Not sure what that is? Check out our blog Exterior Paint vs. Solid Stain)
These questions are all important for you to bring the vision you have for your home to life, but more than selecting the colour or finish, one of the most fundamental questions to ask when you are selecting paint is whether the paint is for the interior or the exterior. Unlike the previous decisions you need to make when selecting house paint, this question is not about style or artistic choice, but about science, which can be overwhelming. Interior and exterior paints serve different purposes, and as a result are engineered differently, in ways that you won’t be able to tell at a glance. For this reason, if you don’t know anything about paint chemistry, you might think that it’s all basically the same. That’s why it’s so important to understand this distinction, because otherwise you might use the wrong type of paint on your home, which would be a big mistake! Using interior or exterior paint where it’s not designed to be used can not only give you a bad result, but can damage your property and be hazardous to your health. But don’t worry, we’re about to break down the fundamentals of paint chemistry, what the differences are between interior and exterior paint, and why you can’t use them interchangeably, so you can be confident that you’re making the right choice for your home.
What’s the Difference Between Interior & Exterior Paint?
In order to understand the difference between interior and exterior paint, you’re first going to need to have a basic understanding of what house paint is as a whole. First of all, paint comes in two categories, oil-based paint and water-based paint, sometimes called latex or acrylic paint. Though oil-based paint used to be the standard for exterior painting, studies found that it is harmful to people exposed to it and to the environment. As a result, the use of oil-based paint is prohibited except in very rare circumstances. Water-based paints are also easier to clean and dry much more quickly, which are added bonuses. With that understood, fundamentally, paint is four ingredients: solvents, resins, additives, and pigments. When combined in different proportions, they change the characteristics of a paint, such as colour, texture, finish and durability, in order to create a paint with the desired properties for the job. The solvents are what make the paint wet. When the paint is in a can, it’s a liquid, but when it is applied to a surface, the solvents are designed to evaporate as the paint dries, making the paint solid. What remains are the resins, additives, and pigments. Of the remaining ingredients, the pigments are what gives a paint it’s colour. They are finely ground particles that are precisely combined to allow the paint manufacturer to create paints with very subtle differences in colour. Additives give the paint different qualities, such as thickening agents, agents that help with better application, or to make it more durable by providing things like mildew resistance. Finally, resins are the things that bind the pigments together and bind the paint to the surface, basically the glue that holds the paint together. Resins are usually made of silicone, acrylic, or epoxy.
Now that we understand the fundamentals, we can begin examining the differences between interior and exterior paints. To start, in both types of paints, the pigments and solvents are basically the same. The pigments give each type of paint their colour in the same way, and the solvents keep both paints a liquid and begin evaporating upon application. The main difference between the makeup of these two types of paints is found in the resins and additives used. As we talked about, resins are what binds the paint to the surface you’re applying it to. The exterior of your home undergoes massive fluctuations in temperature, battering from snow, rain, and wind, and is hit by the sun’s powerful UV rays. In order to withstand all this, the resins used in exterior house paint are softer. As for additives, exterior paints are full of additives that help make it more weather resistant and make it resistant to mold and mildew, which are essential for the exterior of a home. For interior paints, because they don’t need to be able to withstand drastic temperature changes and intense weather, the resins are much more rigid. However, this also makes them stain resistant and easy to scrub and clean, which is more important for the interior of your home than the exterior. In general though, because the exterior of your home goes through so much more than the interior, the additives and resins used in exterior house paint wouldn’t be necessary for the interior of your home. However, in addition to just being excessive, there are other important reasons why you shouldn’t use interior house paint on your home’s exterior, and vice versa.
Can I Use Interior Paint On The Exterior?
Can you use interior paint on the exterior of your home? The short answer is no. Interior paint will not be durable enough to handle heavy rain, snow, and wind as it lacks the additives and resins found in exterior paint that protect your home. Likewise, using exterior house paint for your home’s interior won’t work either. Just as interior paint doesn’t have the additives and resins to properly protect your home and provide a coat that will last, the soft resins in exterior paint smudge easily and are much harder to clean. Interior walls get smudged regularly, coming in constant contact with furniture, children, and pets, so the ability to clean the paint easily is a big plus! However, there is an even more critical reason why you shouldn’t use exterior house paint on your home’s interior, and that’s safety.
Because exterior house paint needs to be heavy duty enough to withstand intense conditions, some additives contain ingredients that can be harmful. These are called Volatile Organic Compounds, or VOCs, carbon-containing chemicals that vaporize at room temperature and are dangerous to inhale. Some examples of VOCs are formaldehyde, benzene, or acetone. The can will tell you the amount of VOCs in the paint. But just as the painting industry stopped using lead-based paint due to the health risks, and as we talked about, more recently has moved away from oil based paints due to the dangerous chemicals they contain like paint thinner, over the years, paint manufacturers have greatly reduced the amount of VOCs in paint. That said, today some exterior paints still contain small amounts of VOCs and should not be used in enclosed spaces.
Even if you paint with a respirator mask and leave the room to dry, even for a few days, the paint can still be harmful. This is due to a process called outgassing. Outgassing is when a paint continues to release VOCs after it has dried. The amount of VOCs emitted by outgassing is far lower than the amount released during painting, but they are still harmful to be exposed to. Usually, outgassing doesn’t last longer than two days, but has been known to continue for years in much smaller amounts. That said, don’t panic! Even if you are using an exterior paint that contains VOCs, the amount of VOCs is small enough that, if used outside, the paint is perfectly safe, it just can’t be used inside. Now to be clear, regardless of what type of paint you are using, indoor painting should always be done in a well ventilated area, it’s just that interior paints do not contain the VOCs found in some exterior paints.
What Happens When I use Interior Paint On The Exterior of My House?
If you try to use interior house paint on your home’s exterior, the first thing you’re going to notice is the application. Resins bind the paint to a surface, and the resins used are very different between interior and exterior paint. This means that interior paint will not apply as easily or have as good coverage when you use it on the exterior. This will also mean that you will likely need more coats of interior paint if you want the coat to be full and solid. That said, once you’ve covered the surface thoroughly and let it dry, you might not notice any problems. In fact, it might even look pretty good! But trust us, this won’t last. Interior paint just isn’t chemically engineered to withstand exterior conditions. I’m not saying that after the first light rain fall the paint is going to wash away like chalk on a driveway, but the paint is not designed to be durable enough to last the continuous exposure to the elements. As well, the temperature of a home’s interior doesn’t vary all that much, so interior paints are understandably not designed to go through the dramatic change between summer and winter. If you use interior paint on your home’s exterior, it will fade from exposure to the sun’s rays, and will quickly bubble, crack, and peel from snow, rain, and fluctuating temperature. As well, if you use interior paint on exterior wood, if the paint is not designed to be used on wood, it will lack the additives necessary to protect the material. Now, instead of just being an aesthetic problem, your steps, deck, or porch could rot and become full of mold and mildew. In addition to being expensive to repair or replace, rotting front steps or porches are a serious safety hazard!
That’s why it’s essential to use the right type of paint for every painting project. With this in mind, it’s also important to understand that not all exterior paints are the same and should not be used interchangeably. Different paints will have different resins and additives based on the material they are designed to be used on. For example, very porous materials like brick and stucco are full of tiny crevices, thus it’s more difficult to apply paint evenly than to smooth surfaces like aluminum siding. As a result, a paint designed for brick is going to have different resins in it than one designed for aluminum. As well, if you are painting a wooden deck, the paint used will need additives in it that prevent rot, mold, and mildew, additives that wouldn’t be necessary in a paint designed for concrete. Always be sure to carefully read the label on a paint can and speak with the professionals at the store before purchasing your paint to make sure that you know exactly what you’re getting.
But if after reading this you’re still finding yourself overwhelmed at the idea of picking the right type of paint, or aren’t confident in your ability to apply it properly once you’ve purchased it, you can always put your interior and exterior painting projects in the hands of professional painters. Having served the Greater Toronto Area for nearly a decade and a half, Encore Painting Ltd. has the knowledge and experience to make whatever painting vision you have a reality. We use only the highest quality house paint, and know exactly what to use in any situation in order to apply an even, lasting coat to your home’s interior and exterior. Let our experts worry about the science of paint chemistry so that you can focus on the fun parts of the job, like selecting colours and planning the new look for your home. View our full list of professional painting services in Toronto to see if Encore Painting is right for you, and don’t hesitate to get a free estimate today.