Almost everyone has a few half-full cans of paint kicking about in their garage.
So, can you mix interior and exterior paint together and use it as one?
You may not be aware that different paint types contain different formulations and chemicals, are made from different substances, and when mixed together can react differently.
There are several more factors you have to consider first so read on to find out whether it is safe and appropriate to do so...
What is the difference between interior and exterior paint?
Paints are designed specifically for their relevant surface type and their level of exposure to the elements.
There are 2 main categories paint falls into - oil-based and water-based. Water-based paints are latex and acrylic, whereas oil-based are sometimes called alkyd.
You can get paints in both categories for use indoors and outdoors. Oil-based paint tends to be more dirt resistant for outdoor applications. However, latex paints are often used instead as the coat is more long lasting.
Exterior paints are designed to resist humidity and temperature fluctuations and tend to be faster to dry.
They are more commonly UV resistant to protect against the sun's rays and may be crack resistant. Additives are often incorporated into these paints for added protection.
Interior paints tend to be water-based. They release fewer fumes than oil-based paints and are safer to use in poorly ventilated areas.
Interior paint is designed to be protected against scuff marks and are often washable.
Pigments, liquids, and binders
These are the other 3 basic elements added to the paint base to create the final product. The pigment is what provides the color and the liquids refer to volatile organic compounds (VOCs).
The binders are used to emulsify the paint into one smooth liquid.
The pigments in interior paints are commonly organic, whereas they are not in exterior paints.
This is because over time, through exposure to sunlight, these pigments are prone to fading.
Exterior paints use synthetic pigments which hold up better to the light but require a longer time between coats.
VOCs are used as a liquid solvent in the paint. Interior paints have to follow federal guidelines and must contain below a certain level of VOCs.
This is because it can have serious health implications if you are exposed to them. These can include headaches and dizziness and may lead to the development of a respiratory disease.
When you are painting the exterior of a home there is a lot more ventilation. This means that exterior paints do not have to adhere to the same VOC guidelines as interior paints.
It is important to look at the VOC content of any paints you plan to use inside your home.
Binders do not only emulsify the paint, but they help it bind to the surface the paint is applied to. They are also used to add extra qualities to the paint.
Interior paint binders are designed to be more resistant to friction and scuffing.
Exterior paint binders are crack resistant, moisture resistant, and can better weather the elements.
What to be aware of when mixing paints
You should ensure that the paints are still usable. Separation is inevitable, so be sure to give the cans a good mix before using.
Use this opportunity to check for any dried paint flakes, debris, or rust in the paints.
You can only mix paints of the same type. This means that oil-based paints can only be combined with other oil-based paints, and water-based with water-based paints.
This is because water and oil will not mix together properly, meaning you are left with a clumpy and inconsistent paint.
Mix the paints really well. If you notice any clumps while mixing you can strain the paint through pantyhose. Store any leftover paint in an airtight container.
We suggest using a large 5-gallon bucket and an electric drill to mix the paints. You will be able to find some long mixer attachments for your drill suitable for this purpose.
It is a good idea to spend a few minutes mixing the paint like this to ensure they are completely combined.
The fumes from VOCs in paint can stay in your home for up to 6 months. During this time you and your family will be breathing them in, which can cause serious health issues.
Always take great care to ventilate the area you are painting in well, and to keep the paint pot sealed when not in use.
Some exterior paints may have chemical additives such as mildewcide incorporated to prevent mold and mildew forming.
Some people are naturally more sensitive to this and using paint containing these additives could cause health problems.
Can you mix interior and exterior paint?
Yes, you can. This is provided they all have the same ‘base’. For instance, an oil-based interior paint could be mixed with an oil-based exterior paint.
If you are to mix paints for interior use, it is important to use a higher ratio of interior to exterior paint.
This will help to keep your walls looking better and protect your family from health issues.
Paint blends for interior use with a higher percentage of exterior paint could potentially expose your family to VOC fumes and mildewcide.
If you are mixing them for exterior use, we recommend using a higher ratio of exterior to interior paint. This will ensure you get the best possible results from your DIY custom paint.
Paint blends used outdoors with a higher percentage of interior paint may not be able to stick to the surface you are painting.
The pigment would fade a lot faster and the paint may crack more quickly.
If you are using a primer underneath the paint, it makes very little difference whether you use one marked interior or exterior.
Where possible, we would always recommend only using paints designed for interior use inside your home. It does not matter if you use interior paint outdoors, you may just need to touch up the paint job more frequently.