Spring will be here before you know it, and it will be time for home improvement projects we’ve all been putting off. If you have a few cans of paint in the garage leftover from your last DIY endeavor, you may be wondering the difference between interior and exterior paint. And can you use interior paint for outdoor projects?
Interior paint has different ingredients and properties than exterior paint, so you shouldn’t use those paint products outside. The environtmenal conditions for indoor surfaces are very different from the harsh conditions paint must withstand when it’s used outdoors. So, using interior paint outside will produce bad results and a paint job that won’t last.
Difference Between Interior and Exterior Paints
Both products come in paint cans, you apply them the same way, and they look the same. So, what’s the difference?
Paints intended for indoor use goes onto walls easily and is designed to withstand general cleaning and household wear and tear. If you have kids or pets, you may want to go with one of the higher quality indoor paint options for added durability. Still, they’re not intended to stand up to weather, rain, UV rays, and more.
There are both latex (water-based) and oil-based options for indoor paint, though latex is most common for all-purpose use.
Indoor paint is made up of five components:
- Base: water/latex or oil
- Pigments: Organic or man-made colors
- Solvents: Quick evaporating liquids to keep binders/pigments suspended
- Additives: To speed drying time, repel moisture/mildew, etc.
- Resins: Binders for durability and hardening
As opposed to exterior paint, indoor paint has different binders, pigments, and resins that are formulated for use inside your home. These paints have properties that make them easy to clean with soap and water or a wet sponge, and they stick to your indoor walls.
Indoor paints typically dry faster (to the touch) though they may take longer to cure completely. They also have fewer volatile organic compounds (VOCs) so there’s less of a health hazard when using them inside your home. Fewer VOCs also means the product is more environmentally-friendly.
Interior paint is also thinner than exterior paint, which makes it easier to apply to your walls and ensures even coverage. That’s why you normally have to do at least two coats when you paint inside your home.
Exterior paint is formulated for use on outdoor surfaces. It’s designed to withstand the harsher environment outside your home and stand up to the elements. In the past, a big difference between interior and exterior paint was that exterior paint was almost always oil-based.
However, that’s changed and now water-based exterior paint is typically preferred. Still, exterior paint is available in both oil-based and water-based options, just like interior products.
Exterior paint is comprised of the same five components as interior paint (base, pigments, solvents, additives, and resins). However, the makeup of those components is different since the paint is intended for a different purpose.
Paint for use outdoors has more additives than indoor paint. That’s because it needs to be able to hold up under harsh conditions. Depending on where you live, the paint might be exposed to rain, wind, snow, ice, temperatures that are below freezing, or temperatures that are over 100 degrees Farenheit.
So, additives are used to help the paint expel moisture and maintain its hard cure. It also needs to be able to keep from cracking or peeling when surfaces expand and contract with temperature fluctuations. By using different resins that are softer and more pliable, the paint can resist chipping, peeling, cracking, and mildew growth.
Exterior pigments also need to withstand the sun’s harmful UV rays without fading or changing color. Using different chemical compounds and formulas, exterior paint can withstand the tough conditions outside much better than interior paint.
Can I Use Interior Paint Outside
The simplest answer is no, you should not use indoor paint outside. Doing so will likely cause more work later on when the paint job doesn’t turn out the way you wanted.
You shouldn’t use indoor paints outside because they don’t have the same additives and chemical properties as exterior paint. While the paint may go on easily, it’s much thinner and would require many, many coats to get sufficient coverage.
Once the paint dried, it wouldn’t be able to stand up to the environmental conditions. Soon, you’d see cracking, peeling, and chipping.
However, there may be some exceptions where you could get away with using interior paint outside your home, like in your garage, shed, or other detached areas sheltered from the elements (assuming you had enough ventilation and could stay away while the paint dried).
What Happens When You Use Interior Paints Outside
1. Interior Paint Won’t Stick
One of the things you’ll notice right away is that your paint may not adhere properly to your outdoor surface.
Interior paints are much thinner than exterior paints and they don’t have the same bonding additives to help them stick to the porous surfaces you find outside. For example, painting onto brick or stone would look runny, and you may see bubbling and seeping on wood surfaces.
2. Paint is Too Thin
Just to get sufficient coverage that doesn’t look streaky and uneven, you would need many coats of interior paint on your outdoor wall or surface. If you normally use 2 or 3 coats to do an indoor wall, you’ll probably have to do twice that on an outdoor wall. Even then, the paint will still be thin and susceptible to damage.
3. Looks Unprofessional
Using interior paint for an exterior job won’t produce the results you were hoping for. Typically it will look sloppy and uneven no matter how many coats you apply. Plus, once the paint is exposed to the sun or other elements for a short time, it will look even worse.
4. Paint Job Won’t Last
These paints are designed for use inside your home. So when you use them outside where there’s rain, sun, ice, wind, mold, and other harsh conditions, it won’t be long before the paint starts breaking down. The color will fade quickly and the paint will eventually crack, chip, peel, and flake.
What is the Best Paint for Exterior Surfaces?
Water-based (latex) paint is the best to use on your outdoor surfaces. Not only has the water-based paint quality increased dramatically over the last two decades, but these products are better for you, your family, and the environment.
Water-based paint has fewer VOCs, which means they’re less toxic. Other benefits include:
- Less odor/fumes
- Less flammable
- Easy to clean up
- No need for paint thinners or additives
- Better color retention over time
However, there may be a few situations where oil-based paint is more appropriate such as on wrought iron, trim, or heavy-traffic surfaces like a porch floor.
I Accidentally Used Exterior Paint Inside: Now What?
If you weren’t paying attention to the label and got started on your indoor painting project using exterior paint, it doesn’t mean you have to scrap the whole thing and start again. Still, there are some steps you need to take and some things you should consider if you find yourself in this situation.
- Stay out of the area until the paint is completely dry, which could take days or even weeks.
- Keep the area well-ventilated using fans, windows, and doors
- Check the manufacturer’s specifications to find out the curing time, which will inform you of when the dangerous fumes will stop emitting
- Wear a protective mask whenever you’re in the room until the curing period ends
- Choose a paint brand with the least amount of VOCs
- Avoid cleaning with abrasive brushes or scaping anything against the wall
However, you should keep in mind that exterior paint may release toxic fumes for weeks or months, which can be very hazardous to your health. To keep your family (and even pets) safe, avoid using exterior paint inside if at all possible.
Can I Mix Interior and Exterior Paint?
Interior and exterior paints are different products and are formulated differently. As long as your two paints are the same base (latex or oil-based), you can mix them, but that doesn’t mean that you should.
When you mix the two paints, the outcome is somewhat unpredictable because of all the different chemicals and additives used. If they don’t mix properly or you don’t get a good result, then you’ve wasted both cans of paint.
Also, you shouldn’t use exterior paint inside even if it’s mixed with interior paint because of the dangerous fumes. And the diluted exterior paint may not be durable enough to withstand the environment outside.
So, it’s really not worth the risk to mix the two paints together.
Paints That Can Be Used Inside and Outside
If you want a versatile paint product that you can use for any project, the solution is hybrid paint. Hybrid paints are water-based but contain alkyd resins like oil-based paints. The result is a hard and durable finish without the dangerous fumes.
So, with a hybrid paint you can get the best of both worlds. They’re perfect for cabinets, bathrooms, and other surfaces that you’d typically consider using oil-based paints on. Here’s an example brand of this eco-friendly paint.
Our team includes many home improvement professionals such as painters interior decorators and architects. Collectively, we have over 30 years of experience.