Using interior paint outdoors is generally not recommended. Interior and exterior paints have different formulations that make exterior paints much more durable for outdoor use. This article explores the key differences between interior and exterior paint, the challenges of using interior paint outside, and when you might be able to get away with it.
The Key Differences Between Interior and Exterior Paint
Interior and exterior paints are formulated differently to serve different purposes. Here are some of the main differences:
The binders, or resins, in exterior paints make them more flexible and water-resistant than interior paints. Exterior paint binders need to withstand expansion, contraction, and moisture from rain, dew, and humidity. Popular binders in exterior paint include:
- 100% acrylic resins – Offer excellent adhesion and water resistance. More flexible than vinyl acrylic blends.
- Vinyl acrylic resins – Blend acrylic resins and polyvinyl acetate for good weather resistance at a lower cost. Not as flexible as 100% acrylics.
- Alkyd resins – Provide good moisture resistance for wood trim. Not as UV-resistant as acrylics.
Interior paint binders focus more on washability and stain resistance. Common interior binders:
- PVA/Vinyl binders – Used in flat, matte, and eggshell interior paints. Not very durable.
- Acrylic binders – Found in some interior paints to improve washability and stain resistance.
- Alkyd binders – Used in oil-based interior enamels to improve moisture resistance.
Exterior paints use more pigments that are bleed- and fade-resistant. Titanium dioxide is a common pigment to reflect UV rays. Other exterior pigments resist fading from sunlight exposure.
Interior paints focus more on coverage and washability. Pigments like iron oxide provide good hiding but little UV resistance. Interior colors will quickly fade if exposed to sunlight.
Exterior paints also include additives to boost weather resistance:
- Fungicides/mildewcides – Prevent exterior paints from fungal/algal growth.
- UV absorbers/inhibitors – Absorb UV rays to reduce paint fading and degradation.
- Anti-microbials – Inhibit mold, mildew, and bacterial growth on the paint surface.
These additives are not found in interior paints. Using interior paint outside exposes the paint to organisms and UV rays that quickly compromise its appearance and performance.
Most exterior paints only come in low sheen options:
- Flat – Provides good coverage with a matte look. Not very reflective.
- Satin – Slight sheen. Resists moisture and stains better than flat.
- Semi-gloss – Most reflective/glossy exterior option. Easiest to clean.
Higher sheens offer better stain and moisture resistance for exterior elements. Interior paints come in higher-gloss enamel options that lack weather resistance.
Synthetic resins, fade-resistant pigments, and protective additives all make exterior paints much more durable against outdoor conditions than interior paints. Durability is measured in:
- Abrasion/Scrub resistance – Exterior paints are tougher against scuffs and scrubbing.
- Flexibility – Exterior resins expand and contract with temperature changes without cracking or peeling.
- Moisture resistance – Repels rain, dew, and humidity better.
- UV resistance – Pigments and additives prevent fading and degradation from sunlight.
Interior paints simply cannot withstand years of sun, rain, temperature swings, etc. Without exterior additives, they quickly lose integrity.
The Problems With Using Interior Paint Outside
Using interior paint outdoors may seem like an easy way to save money or use up leftover paint. But there are several significant issues with this:
The sun’s harsh UV rays will quickly cause interior paint colors to fade. While all paints fade over time outdoors, interior paints have little UV protection. Fading can happen in as little as 2-3 months.
Darker colors like reds, blues, and browns are especially prone to fading since they absorb more heat. Light tints like yellows and pinks may last slightly longer. But all interior paints will eventually fade outside.
Fading happens because the sun’s UV radiation degrades exterior paint polymers in a process called photodegradation. Pigments also degrade and lose color. Interior paints offer almost no resistance to photodegradation.
Without flexible resins designed for expansion and contraction, interior paints are prone to flaking and peeling outdoors. This happens as temperatures cause painted surfaces to continually expand and contract.
Moisture getting under interior paint that lacks water resistance can also cause bubbling and peeling as the moisture tries to evaporate. This moisture can come from rain, dew, melting frost, sprinklers, etc.
Chalking often precedes peeling as the binders deteriorate. The paint surface gets a chalky, powdery look before flaking off.
Pigments designed for hiding rather than UV resistance mean interior paints get less coverage per coat outdoors. It often takes 3 or more coats to get decent coverage with interior paint outside.
The thinner consistency of interior paints also contributes to poorer coverage on textured and porous exterior materials. Exterior paints have thicker consistencies that help fill surface irregularities.
Growth of Mold, Mildew, and Algae
The damp conditions outdoors expose exterior paints to mold, mildew, fungi, and algae growth. Interior paints lack the additives that prevent this biological growth.
Mold and mildew appear as black spots that can eventually discolor the entire paint surface. Algae and fungi create unsightly streaking on exterior paints without antimicrobial additives.
These growths are not just aesthetic issues. Mold and mildew can potentially deteriorate surfaces and pose health risks for those with allergies or compromised immune systems.
The cumulative result of fading, peeling, poor coverage, and microbial growth means that interior paints will only last a year or two outdoors, at most. In contrast, high quality exterior paints can easily last 7-10 years before needing to be repainted.
The short lifespan of interior paint outdoors ends up costing more as surfaces require more frequent repainting. Taking the time to use exterior paint saves money and effort in the long run.
Can Interior Paint Work As a Primer?
Some sources recommend using interior latex paint as a primer coat for exterior surfaces, followed by a topcoat of true exterior paint. The logic is that a coat of interior primer may help the topcoat last longer.
However, this is still generally not recommended by paint manufacturers:
- Most exterior paints are self-priming. An additional primer coat is unnecessary.
- Interior primers like PVA lack the adhesion strength to properly prime exterior surfaces. The topcoat may peel or delaminate.
- Moisture penetration through the interior primer can still cause the exterior topcoat to fail prematurely.
- Light tints of interior paint used as a primer can negatively affect the color and hiding of the topcoat.
A dedicated exterior primer designed for the specific surface is always preferable to interior paint primers:
- Wood – Use an exterior wood primer to seal knots, tannins, etc.
- Masonry – Choose an alkali-resistant exterior primer.
- Metal – Prevent rusting with a corrosion-resistant exterior primer.
If you need to use a primer, spend a few extra dollars on a high quality exterior-grade option instead of trying to make interior paint work in this application.
When Interior Paint Might Work Outdoors
While interior paint is not ideal for outdoor surfaces, there are a few scenarios where you may be able to get away with using it outside:
Short Term Projects
If you need a finish to last a few months over one season for a short-term project, interior paint may suffice. Just don’t expect it to last beyond a year.
This can work for temporary touches like:
- Holiday/seasonal decor that gets displayed for a couple months then stored.
- Quick exterior repairs that will get a full repaint later.
- DIY furniture destined for a seasonal screened porch or patio.
Using interior paint on exterior surfaces that have good coverage from the elements can extend its lifespan. Areas like:
- Covered front porches.
- Under roof eaves.
- Garage interiors.
- Screened pool enclosures.
Rain, direct sun, temperature swings, etc will still limit its durability in these locations. But interior paint may last 1-2 years.
Applying a clear exterior-grade sealant or protectant over interior paint can help it survive longer outside:
- Urethane sealants provide the best protection.
- Exterior varnishes/lacquers also seal the paint.
For small exterior paint touch-ups needed between full repaints, interior paint can serve as a short term solution if matched well. Just don’t expect flawless uniformity or durability.
Touching up siding, shutters, gutters, fencing, etc. with leftover interior paint may tide things over for a few months before a full repaint. Make sure to clean and prep the surface first.
Match sheens as closely as possible. Flat interior paint tends to stick out more than satin, semi-gloss, or gloss exterior paints.
Interior paint touch-ups will likely fade faster than the existing exterior paint, leaving noticeable differences. Feather out the edges for a smoother transition.
For children’s crafts, kids’ rooms, school projects, and other non-critical surfaces, interior latex paint can provide ample durability and save money.
Some ideas where interior paint may work sufficiently for kids:
- Wooden swingsets
- Outdoor cardboard sculptural projects
- Fences around sandboxes
- Temporary chalkboard paint on plywood
Have kids focus on enjoying the painting process and being creative rather than worrying about longevity. Supervision is still required for safety.
For long term kids’ projects, use exterior grades of chalkboard or magnetic paint. And seal cardboard creations with exterior varnish.
Indoor/Outdoor Carpet Paint
Interior/exterior carpet paint exists for refreshing old indoor/outdoor carpeting. The paint levels out worn areas and provides UV resistance.
While marketed as interior/exterior, these carpet paints are designed to handle the elements better than standard interior paints.
They come in several exterior-friendly colors to rejuvenate faded old carpeting. Allow the painted carpets to dry fully before use.
Some premium interior paints may outperform budget exterior paints, especially if top-coated. Quality interior acrylic or alkyd paint provides better binders, pigments, and coverage.
Still use exterior precautions like sealing, maintenance, and touch-ups. And limit high-traffic/friction areas. High quality interior paints simply have a better chance of withstanding elements.
But a premium exterior paint designed for durability will always be the best choice for siding, decks, fences, outdoor furniture, etc.
Low Sunlight Areas
A heavily shaded location out of direct sun may prolong the lifespan of interior paint outdoors. Without intense UV exposure, paint will experience less photodegradation.
North-facing surfaces, and areas shaded by trees, overhangs, etc. are good candidates for extended interior paint life. Just beware of moisture damage.
Out of direct sun does not mean interior paint will last as long as exterior paint. But it improves chances in a protected low sunlight location.
Oil-Based Interior Paint
While latex interior paints fail quickly outside, oil-based interior enamels may last longer due to their moisture resistance. Still expect fading and deterioration in months.
Oil-based interior trim and door paints, or alkyd-based garage floor coatings, can survive outdoor conditions better than latex interior wall paints. Their durability is closer to exterior paints.
Paint durability depends on oil-type:
- Linseed oil – Provides the most protection outdoors.
- Alkyd – Based on synthetic oils. More flexible than straight linseed oil.
- Latex oils – Least durable oil-based option outdoors.
Follow safety precautions when using oil-based paints. Disposal must follow hazardous waste guidelines.
Key Considerations When Using Interior Paint Outside
While interior paints are not ideal for outdoor use, if you do need to use interior paint for a specific outdoor project, keep these tips in mind:
- Test in an inconspicuous spot first to ensure acceptable durability.
- Only use high quality interior paints – cheap paints fail the fastest.
- Apply multiple coats for better coverage and protection.
- Use flat or satin finishes – glossier paints show flaws more as they deteriorate.
- Sealing with an exterior-grade clear topcoat adds protection.
- Maintain the paint by checking for fading, peeling, etc. and touching up as needed.
- Expect to repaint frequently – don’t assume interior paint will last outside.
- Avoid areas of direct sun exposure for maximum longevity.
- Keep moisture away as much as possible – rain, sprinklers, etc.
- Consider removable paint options like chalk paint if you need to redo the finish frequently.
- Follow all safety precautions if using oil-based interior paints outdoors.
Interior paint may suffice outside for temporary low-use projects. But for anything permanent or high-traffic, use exterior paint specifically designed to handle sun, rain, and temperature swings for long-lasting results.
Frequently Asked Questions
Can I use interior house paint on outdoor wood?
It’s not recommended. Interior paints lack UV blockers and exterior-grade resins that prevent fading, peeling, cracking, etc. For outdoor wood projects, use an exterior-grade wood paint for protection.
What’s the difference between interior and exterior paint?
Exterior paints have fade-resistant pigments, acrylic/alkyd resins, fungicides, and UV blockers that make them far more durable against outdoor exposure than interior paints.
How long does interior paint last outside?
At best, interior paint may last 1-2 years outside with fading and deterioration. In direct sun and rain, it can fail in months. Exterior paints can last 5-10 years outdoors.
Can I use interior paint as a primer for exterior surfaces?
It’s best to avoid this and use a dedicated exterior primer. Interior paint lacks the adhesion and moisture resistance needed for proper exterior priming.
What kind of interior paint is best for outdoor use?
Higher quality acrylic and oil-based interior paints have a better chance of withstanding outdoor exposure than cheap latex paints. But exterior paint is still the best choice.
Can I mix interior and exterior paint?
It’s not recommended as it compromises the exterior paint’s durability. Both latex and oil-based interior and exterior paints should not be mixed or interchanged.
Interior paints simply lack the binders, pigments, additives, and durability needed for long lasting protection on outdoor surfaces. While interior paint may work temporarily in certain low-exposure cases, exterior paints specifically designed for the elements are always the best choice for outdoor projects. Investing in quality exterior paint saves time, money, and frustration down the road.