Most homes need a new coat of paint from time to time, either on the inside or the outside. It is a project that many people take on, and you may be considering it yourself. But what is the difference between exterior paint and interior paint? We've researched for you, so you can be sure you know the correct paint to use for your project.
Interior paints have different additives and resins than exterior paints have. The resins in exterior paints must remain softer and more pliable since there is a greater amount of expansion and contraction due to weather. Resins used in interior paints tend to be harder, making them more durable and able to withstand scrubbing and scuffs. It is also important to know that additives and resins in interior paints often exude fewer harmful vapors than those used in exterior paints.
We discuss the differences between interior and exterior paints in more detail below. We'll also discuss the reasons for and against using oil-based and latex paints, so please keep reading.
Interior Vs. Exterior Paint
Painting is a very DIY-friendly project for a home. It is important to use the proper paint for the project, though. So whether you are painting your bedroom or the outside window trim, you need to know what the correct paint is and what the differences are.
The primary differences between interior and exterior paints are the resins and additives. We've already discussed that the resin for an exterior paint needs to remain softer and more flexible to allow it to withstand the weather, but there are several other differences that are important.
Exterior paint sits out in the weather. It has to withstand the cold and wet weather as well as it does the hot and dry. For this reason, it uses a resin base that remains somewhat soft. This allows the paint to adhere during the expansion and contraction that comes with this heat cycling. But that soft resin that handles the weather well does not hold up as well to scuffs and cleaning like it needs to inside the home.
The exterior paint also usually contains additives and resins that provide several benefits, such as:
- Mildew and mold resistance
- Added flexibility
- UV resistance
- Chip resistance
- Moisture resistance
- Improved adhesion
But the various resins and additives used in outdoor paint often give off a larger amount of harmful fumes. This is especially troublesome in an enclosed area, such as inside your home.
Both latex and oil-based exterior paints are available. While most people use latex on siding, oil-based paint is still often used for trim and doors because of the extra durability it offers over latex.
Here are a few excellent exterior latex and oil-based paint products:
Prestige Latex Flat White Exterior Paint And Primer
Rust-Oleum Oil-Based Satin Cranberry Door Paint
The resins and additives used in interior paint need different qualities than those in exterior paint. For the most part, interior paint does not need to hold up to drastic temperature changes, and UV is not as much of a problem. Mold, mildew, and moisture aren't usually much of a problem inside.
What is a problem on the interior, though, is food splatters, finger touches, scuffs, and bumps. Because of this, interior paint tends to contain additives and resins that make it easier to clean and more resistant to scuffs.
Interior paint is also formulated to give off less harmful fumes than exterior paint. This is important since you will spend a lot of time enclosed with the same air as interior painted surfaces. Most interior paint already naturally produces fewer fumes and contains fewer VOCs (volatile organic compounds) than exterior paint does. Many interior paints are designed and marketed as low or zero VOC paints, making them even safer still.
It is because of these harmful fumes that most people only use latex paint on the interior. Oil-based paint tends to produce more fumes. This is especially true during the painting process and right after.
Here are some examples of typical interior latex paints:
Prestige Latex Dove Gray Interior Paint
Rust-Oleum Simply Home Pure White Latex Interior Paint
Oil-Based Vs. Latex
We've already mentioned that oil-based paint produces more fumes. Another downside with it, aside from the higher fumes, is that it tends to yellow and crack with age. On top of that, to clean it up, you need to use something like mineral spirits or solvent, such as these:
Klean-Strip Odorless Mineral Spirits
Sunnyside Paint Thinner
There are some benefits to oil-based paints:
- Harder surface than latex
- Slower drying
- Smoother finish
- Higher moisture resistance
- Better adhesion
Latex paint is good enough now that higher-quality latex has nearly caught up to oil-based paint in these areas, though. The only area they don't do nearly as well on is slower drying, which is only a benefit in cases where you need to work slowly for some reason. Most people prefer a faster drying time.
Other benefits of latex paint:
- Not usually flammable
- Doesn't yellow
- Lower odor
- More flexible once cured
You can usually do just fine with latex paint, and even most professionals don't use oil-based paint anymore. The only places that many people ever use oil-based paint are high contact areas, such as doors and trim areas. In those cases, the more durable smooth surface and better adhesion that oil paint offers make up for the difficulties of working with it to some people.
Should You Use Exterior Paint In The Garage?
You should not use exterior paint on the inside walls of your garage. The better and safer choice for this area is interior latex paint.
Often times exterior paint contains resins and additives that give off harmful fumes. This can make it dangerous in an indoor area. People often use their garages for more than storing their cars. It is common for people to spend a lot of time in their garages, doing crafts, building furniture, or working on their cars. That makes using exterior paint in this enclosed area very dangerous and harmful, unlike when you use it on an exterior area with good ventilation.
One of the reasons for additives in exterior paint is to add weather resistance. While it may get cold insider your garage, this area is usually not exposed to wind, rain, and sun. Because of this, interior latex should hold up well inside a garage.
Can You Mix Exterior Paint With Interior Paint?
As long as paints are both latex or both oil-based, you should be able to mix interior and exterior paints. What you end up with may not be desirable, though.
Generally speaking, if you mix a small amount of interior paint into exterior paint it will not have much effect. It may degrade the durability, but you still basically have an exterior paint overall. The higher the percentage of interior paint in the mixture, the worse it is going to hold up to weather, UV, and mold. You degrade the benefits of the resin and additives that make the exterior paint what it is.
If you reverse that and instead add exterior paint into an interior one you add at least some, maybe a lot, of the downsides of exterior paint into your interior. So you may have a certain level of harmful fumes that exterior paint often produces, and you may wind up with a paint that does not hold up to scuffs and clean-ups as well as interior paint usually does.
Added to the possible problems mentioned above, you also may not be able to match your mixed concoction in the future, should a touch-up ever become necessary.
Is It Bad To Use Exterior Paint Indoors?
You should not use exterior paint indoors. Exterior paint contains resins and additives that may give off fumes. That is the biggest problem in doing this. Even if you don't smell the chemicals, they can still be there. Some exterior paints may continue to be harmful for over six months after application.
There are other problems you may encounter when using exterior paint inside. It will expand and contract with the weather, but it will not hold up to the kind of scuffs and wear from frequent touching that is likely inside the home, for example. It's best to save the exterior paint for the exterior of your home.
While there are a lot of similarities between interior and exterior paints, there are some important differences. Interior and exterior paints are each specifically designed for the intended use, and you will not get good results if you use interior paint outside or exterior paint inside.
On top of that, it can be dangerous to use exterior paint inside. You should always use paint in the proper locations, meaning exterior paint outside and only interior paint inside of your home.
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