Have you ever found yourself halfway through a painting project, only to discover you’re running low on paint? That leftover can of interior paint in the garage starts looking mighty tempting when your exterior project needs just a little more coverage. Or perhaps you’re wondering if mixing different paint types could create the perfect custom color or finish for your specific needs. The question “Can I mix exterior and interior paint?” is more common than you might think, and today we’re going to thoroughly explore this topic to help you make informed decisions about your painting projects.
Understanding the Key Differences Between Exterior and Interior Paint
Before discussing whether these paints can be mixed, it’s essential to understand what makes them different in the first place.
Composition and Formulation
Exterior and interior paints are specifically formulated for their intended environments. Their chemical composition varies significantly to address different challenges:
- Exterior Paint: Formulated to withstand harsh weather conditions including rain, snow, UV radiation, temperature fluctuations, and pollution. Contains more binding resins and additives like fungicides and mildewcides to prevent growth of mold and mildew on outdoor surfaces.
- Interior Paint: Designed for indoor use with lower VOCs (Volatile Organic Compounds) for better indoor air quality. Contains different additives focused on stain resistance, scrubbability, and providing an aesthetic finish rather than weather protection.
Durability Factors
The environments these paints must endure require different durability characteristics:
- Exterior Paint: Must remain flexible to expand and contract with temperature changes without cracking or peeling. Contains UV blockers and more elastic polymers to resist fading and chalking.
- Interior Paint: Formulated for abrasion resistance, easy cleaning, and colorfastness under artificial lighting. Prioritizes appearance and performance in controlled indoor environments.
VOC Content and Safety
Safety considerations are another significant difference:
- Exterior Paint: Typically contains higher levels of VOCs and stronger chemicals that would be inappropriate for confined indoor spaces. These chemicals help the paint withstand outdoor elements.
- Interior Paint: Specifically formulated with lower VOCs and less harsh chemicals to ensure it’s safe for enclosed spaces where people spend most of their time.
Can I Mix Exterior and Interior Paint Together?
Now to address the central question of whether these two types of paint can be mixed.
The Short Answer
Yes, technically you can mix exterior and interior paints of the same base (water-based with water-based, oil-based with oil-based), but it’s generally not recommended by paint professionals for several important reasons.
What Happens When You Mix Them?
When you combine exterior and interior paints, several issues can arise:
- Compromised Performance: The resulting mixture will have neither the full weather protection qualities of exterior paint nor the aesthetic and low-VOC benefits of interior paint. You essentially create a “middle ground” product that excels at nothing.
- Unpredictable Results: The chemical additives in each paint type may interact in unexpected ways, potentially causing problems with adhesion, drying time, or finish quality.
- Inconsistent Appearance: Differences in sheen levels and pigmentation can lead to uneven coloration, streaking, or an inconsistent finish.
- Reduced Durability: The protective qualities of both paints become diluted, potentially leading to premature failure of the paint job.
Scenarios Where People Consider Mixing Paints
Let’s examine some common situations where people might consider mixing these paint types.
Running Out of Paint Mid-Project
If you’re halfway through painting your deck and run out of exterior paint, mixing in some interior paint might seem like a quick fix. However, this approach can lead to sections with inconsistent durability and appearance.
Better Alternative: Purchase more of the same exterior paint. Many paint stores can match your existing paint color perfectly.
Creating Custom Colors
Some DIYers might consider mixing different paint types to achieve a specific color that isn’t available off the shelf.
Better Alternative: Ask your paint retailer to tint either exterior or interior paint to your desired color. Modern paint mixing technology can match virtually any color in either paint type.
Trying to Save Money
Using up leftover paint by mixing types might seem economical.
Better Alternative: Use appropriate paint for each application. The cost of redoing a failed paint job far exceeds the savings from mixing leftover paints.
What Happens If You Paint the Exterior with Interior Paint?
Using interior paint outdoors is a common mistake with predictable consequences.
Short-Term Issues
Initially, interior paint applied outdoors may look acceptable, but problems will emerge quickly:
- Faster fading and discoloration due to lack of UV protection
- Poor water resistance leading to bubbling and peeling
- Vulnerability to mildew and mold growth without antimicrobial additives
Long-Term Consequences
Over time, the situation worsens:
- Significant peeling and flaking as the paint fails to expand and contract with temperature changes
- Complete paint failure requiring thorough removal before repainting
- Potential damage to the underlying surface from moisture intrusion
Can I Use Exterior Paint Inside?
The reverse scenario—using exterior paint indoors—presents different challenges.
Health and Safety Concerns
Using exterior paint indoors can pose health risks:
- Higher VOC content can cause respiratory irritation and headaches
- Stronger odors that may persist for longer periods
- Potential long-term exposure to chemicals designed for outdoor ventilation
Performance Issues
Beyond health concerns, there are practical considerations:
- Exterior paint may have a different sheen or appearance under indoor lighting
- Some exterior paints may remain softer indoors where they don’t experience the same curing conditions
- Potential for stronger odors that linger longer than interior formulations
When It Might Be Acceptable
There are limited scenarios where using exterior paint indoors might be justified:
- In rarely used spaces with excellent ventilation, such as garages or storage areas
- For specialty applications requiring extreme durability
- In areas with unusual moisture concerns, like laundry rooms (though specialized interior moisture-resistant paint would be better)
Can I Mix Exterior and Interior Paint for Wood?
Wood surfaces present specific considerations when it comes to paint application.
For Outdoor Wood Surfaces
Outdoor wood requires the protection that only exterior paint can provide:
- Protection against moisture absorption and UV damage
- Resistance to mildew and rot
- Flexibility to accommodate wood’s natural expansion and contraction
Mixing in interior paint would compromise these essential protective qualities.
For Indoor Wood Surfaces
Indoor wood has different needs:
- Scuff and stain resistance for furniture and trim
- Appropriate hardness for durability without cracking
- Aesthetic quality under indoor lighting conditions
Using a mixture would likely result in suboptimal performance for indoor applications as well.
Is It Okay to Mix Different Types of Paint?
Beyond just interior and exterior distinctions, there are other aspects of paint compatibility to consider.
Water-Based vs. Oil-Based
The most fundamental rule in paint mixing is maintaining base compatibility:
- Water-based (latex/acrylic) paints should only be mixed with other water-based paints
- Oil-based paints should only be mixed with other oil-based paints
Mixing across these bases will result in immediate separation and ruined paint.
Different Brands
Mixing different brands of the same type of paint (e.g., two different brands of exterior latex paint) is generally less problematic, though still not ideal:
- Different manufacturers use proprietary formulations
- Performance characteristics may vary between brands
- Color matching may not be precise even with seemingly identical colors
Different Sheens
Mixing paints with different sheen levels (flat, eggshell, satin, semi-gloss, gloss) will result in an inconsistent finish:
- The final sheen will be somewhere between the original sheens
- Application may be uneven, resulting in patches of different reflectivity
- The aesthetic appeal will likely be compromised
Professional Recommendations
Here’s what paint industry professionals typically recommend:
For Leftover Paint
Instead of mixing different types of paint:
- Use interior leftovers for interior touch-ups or smaller projects
- Use exterior leftovers for exterior maintenance or small outdoor items
- Donate usable paint to community organizations or theater groups
- Dispose of old paint properly according to local regulations
For Color Matching
If you need a specific color:
- Bring a sample to your paint retailer for professional color matching
- Have them mix the right type of paint (interior or exterior) in your desired color
- Purchase a small sample first to test the color before committing to larger quantities
For Budget Concerns
If cost is a factor:
- Look for sales or rebates on quality paint rather than compromising with mixed types
- Consider lower-cost lines from reputable manufacturers instead of mixing
- Remember that labor costs and time typically exceed paint costs in most projects
When Mixing Paint Might Be Acceptable
While mixing exterior and interior paint is generally discouraged, there are limited scenarios where mixing same-type paints might be acceptable:
- Mixing two interior latex paints of different colors to create a custom shade
- Combining two exterior latex paints for the same purpose
- Using mixed paint for temporary applications, art projects, or where longevity isn’t critical
In these cases, ensure the paints share the same base (water or oil) and are relatively fresh.
Final Words: Making the Best Decision for Your Project
While it might be tempting to mix exterior and interior paints to use up leftovers or create a custom formulation, the potential problems outweigh the benefits in most situations. Paint manufacturers spend considerable resources developing specialized formulations for specific applications, and compromising these formulations typically leads to disappointing results.
For the best outcome in any painting project:
- Use the right paint for the right purpose
- Purchase appropriate quantities to minimize leftover paint
- Store unused paint properly to maximize its usable life
- Seek professional advice when uncertain about the best product for your needs
Remember that the success of your painting project depends not just on application technique but on starting with the right materials. Cutting corners on paint selection often leads to premature failure and the need to repeat your work sooner than expected.
By understanding the differences between paint types and using them as intended, you’ll achieve professional-looking results that stand the test of time, whether you’re painting indoors or out.
