Interior paint is used for indoor areas like walls and ceilings, while exterior paint is used on surfaces like siding and fences outside the house. If you have exterior paint left over from an outdoor project, you might wonder, "Can I use exterior paint inside??

Generally, exterior paint should not be used inside, mainly due to health concerns and because its qualities are incompatible with indoor spaces.

VOCs

Exterior paint contains far more VOCs (volatile organic compounds) than interior paint. 

VOCs are chemicals in paint and other home materials, such as cabinets, furniture, carpeting, and flooring. VOCs produce toxic gasses and can potentially harm residents and the environment if the levels are high enough.

While the VOCs will eventually disperse, this could take days or weeks. Some VOC-laden substances off-gas for years.

If you use exterior paint inside, you are releasing up to 17 times more VOCs into the home than if you use interior paint. 

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Compatibility

While exterior paint is marginally compatible with indoor spaces, it isn't the best paint to use here. Interior paint is formulated for indoor areas.

• Exterior paint: Due to extreme outdoor conditions such as rapid temperature spikes, exterior paint is formulated to expand and contract. But it is fairly soft and does not withstand scratching well.
• Interior paint: Interior paint is more rigid than exterior paint. Its rigidity helps it withstand abrasion from scuffing, damage, and cleaning. 

Spread Rate

Exterior paint has a lower spread rate than interior paint, so you'll need to use more exterior paint than interior paint to cover the same amount of space.

On average, one gallon of water-based exterior paint covers 300 square feet, while a gallon of interior paint covers 375 square feet.

So, four gallons of interior paint will cover 1,500 square feet. But if you use exterior paint, you must add an extra gallon to paint the same space.

Drying Time

Exterior paint has a longer drying and recoat time than interior paint.

Both exterior and interior paint dry to the touch in about one hour. However, the recoat time for exterior paint can be as long as four hours, and the recoat time for interior paint can be from one to two hours.

Ample ventilation speeds up paint's drying and recoat time. When you factor in the limited ventilation of interior spaces, exterior paint applied here would take the full four hours or more before another coat could be applied.

Using Exterior Paint Inside

Though exterior paint should not be used inside the home, it is acceptable in a limited number of interior spaces. 

You could use exterior paint inside a garage, shed, workshop, or other outbuilding that isn't used as a living space.

That's only if you're looking to use up extra paint. If you have a choice, purchase interior paint for indoor spaces.

Using Interior Paint Outside

Interior paint used outdoors doesn't present the same health hazards as exterior paint on the inside. There are fewer VOCs and no chance of them being trapped outdoors.

Using interior paint outside is a poor idea because:

• It will crack since it's not designed to expand and contract with the elements.
• It contains no fungicides and other additives found in exterior paint.
• Interior paint's color will fade faster than exterior paint's color.

Paint to Use for Interiors

Generally, interiors should be painted with 100-percent water-based acrylic-latex paint (sometimes called latex paint or acrylic paint).

The paint base will be clearly labeled as interior paint. When requesting mixed paint from the paint counter, specify that you want interior paint. 

Within the acrylic-latex paint family, choose a gloss that works best for each room of the house. Satin, eggshell, and semi-gloss paint glosses are tough and easy to wipe down, so they work well for kitchens. Bedrooms can accommodate glosses on the flatter end, such as eggshell or flat/matte paint.

Paint to Use for Exteriors

Use 100-percent water-based acrylic-latex paint on house exteriors. This paint, labeled as exterior paint, works for most house siding and trim.

Consider using oil-based enamel paint on exterior doors, trim, and metal for extra protection. Oil-based exterior paint takes a long time to dry and must be cleaned with solvents, not with water. But it provides extra-strong, long-lasting protection from scuffs, rust, and extreme weather conditions.