The paint used for the interior of your building and the paint used for the exterior might look very similar, but there are some significant differences.
Both paints are created to withstand the elements they are up against. Exterior paint must do well in extreme heat and still hold up against the rain. Interior paint will take marks and scuffs and needs to be easily cleaned.
Different needs mean different paints, and while they look very similar, they are, in fact, very different at a chemical level. Let’s take a closer look.
What Is In Paint?
Before we get to the differences, we need to know a little about what constitutes the basic building blocks of any paint.
You have the solvent, the wet part of the paint. This is often water but could be mineral spirits or another liquid. When the paint dries, this liquid evaporates and leaves the color on the wall.
This brings us to the pigment. This is the color of the paint. When the liquid evaporates, the pigment is what colors the remaining ingredients.
One of which is the resin(s). This is what binds the pigment (and additives) to the wall.
As you might have guessed, the last ingredients are additives. This is one of the most ingredients for indoor vs. outdoor paint as the additives can do a lot, such as protect against mold and mildew, aid with cleaning, and more.
Are There Any Similarities Between Interior & Exterior Paint?
When it comes to painting, the pigments are usually the same across the board. Color is a color, and the color needed for a surface is dictated by personal choice rather than needs. Blue paint will not act differently on an exterior wall than red paint. And, there are no colors you would not use inside or out.
The solvents (the liquids that dilute the paint) are also used in interior and exterior paints. But, while oil-based paints can be used for interior surfaces, they are usually not. Oil-based paint has a more pungent smell, and the surface can be harder to clean when oil-based paint is used. Two reasons not to use this type of paint indoors.
The Main Differences Between Interior & Exterior Paint
Resin
One of the main differences between the two types of paint is the resin used. The resin is what binds the color to the surface, and exterior paint has very different needs from interior paint, and the resin has to be able to fulfill those needs.
The temperature of an interior surface is unlikely to face major temperature changes. An exterior wall, however, might have to withstand extreme heat and cold. The paint needs to contract and expand over the year, and exterior paint will have resins that can handle this.
Interior paints will get scuffed and marked, so the resins used need to be more durable and not easily scratched. That durability is also going to make them easier to clean repeatedly.
Additives
The additives are what give the paint its qualities. The solvent gives it body, the pigment gives the color, and the resin holds it to the surface. The additives are what give the paint everything else.
A popular additive in exterior paint stops fading. Exterior surfaces will be in direct sunlight, and the UV light will bleach and fade the color. Interior paint does not need this additive, but exterior paint does.
Additives can also help reduce mold and mildew, a must for exterior paint in humid climates. This additive will mainly be used for exterior paints but can be added to interior paints used for bathrooms and kitchens when there is excess moisture.
Exterior paint also has additives that help the resins adhere to rough surfaces. Most interior surfaces are smooth, but exterior surfaces are often not. When you need to apply paint to rough exterior surfaces, it is the additives that create that extra hold and bond.
Finish
You can find interior paint in many different finishes. Interior paint comes in matte, eggshell, satin, semi-gloss, and gloss. When painting kitchen cabinets, high-gloss paints give a fantastic finish, protect the surface, and make cleaning easier.
Exterior paint does not come in as many finishes. You will have more matte finishes at your disposal and a few gloss options, but not as many as with interior paint.
Cost
Exterior paint costs more than interior paint, which will rise with the number of additives. Exterior paint needs more durability, moisture resistance, fade resistance, and more. This extra ‘stuff’ means more cost.
Exterior paint uses more oil-based solvents compared to interior paint. That is another ingredient that drives up the price.
Of course, interior paint is more expensive than exterior paint, which can be down to the quality. Invest in high-quality paint if you want it to last for longer, and save you money in the long run.
Final Thoughts
While there are many differences between interior and exterior paint, the main one is the additives. This is what allows the paint to do the extra ‘stuff.’
Exterior paint must be more durable, resistant to mold and mildew, and hold up to extreme temperatures. It often uses oil- and spirit-based solvents instead of water, and the resins used to adhere the paint to the wall will change based on the ingredients used and the needs of the paint.
From this, we can say that interior and exterior paint are not interchangeable. While one can be used in place of the other, you might end up with paint that does not hold up to the elements outdoors or does not give you the finish you want for your interior wall.
As with anything, consult with your local professional if you ever need any paint advice.