Are you planning to use some leftover paint from the living room or basement on some patio furniture, or to repaint an exterior wall or door? Wondering can you use interior paint outside? Although the short answer is yes, it’s not recommended.
Interior paint isn’t designed to withstand rain, snow, wind, UV, mildew, or temperature extremes. It has less pigment and binding resins than outdoor paints, and the finish is too porous .As a result, the paint will fade, powder or chalk, crack and flake more quickly.
In this article, we’ll take a look at interior paint, what it is made of, and why it is best suited for use indoors. We’ll also discuss what to do if you do use it outside. By the end of the article, you should have a better understanding of what interior paint is and why it is better suited for indoor use.
What Is Interior Paint?
Interior paints are specially formulated to be used indoors. They are designed to adhere to different surfaces, reflect or absorb light, and withstand abrasion and wear of daily contact and cleaning. They also don’t normally have to withstand the expansion and contraction of the freeze-thaw cycle or exposure to the harsher elements of nature.
Paints are made up of four parts – the base or liquid, additives, pigments, and binders. The binders, pigments, and additives are what make interior paint different for exterior paint, and why they are not interchangeable. Interior paints are designed to have lower volatile organic compounds (VOCs) to minimize health risks due to off-gassing. The VOCs are the solvent that helps keep the liquid from hardening; they vaporize as the paint dries.
Interior paints can be water-based, which includes latex and acrylic paints, or oil-based such as alkyd paints. Water and oil-bases offer different adhesive properties and use. Both are available in different sheens from high-gloss, semi-gloss, to flat. If you’re unsure if something is painted with an oil or water-based paint, wipe it with a cloth dipped in denatured alcohol. If it doesn’t pick up any color, it’s likely oil-based; if it does, it’s water-based.
You can paint over oil-based with water-based or latex paints after applying an oil-based primer or stain lock sealer. Oil-based paints can be used over water-based paints once they are dry – you may need to scuff glossy surfaces gently with sandpaper.
Water-based or latex paints consist of pigments and binders blended in water as the carrier. They dry more quickly as the water evaporates, leaving the color behind. Latex painted surfaces are easier to clean and hold their color better than oil-based alternatives. The paint usually has less VOCs, so it is more environmentally friendly than oil-based paints.
When dry, interior latex paints provide a flexible and durable paint and are ideal for painting ceilings and walls. Many prefer water-based paints for the ease of cleaning brushes, tools, and spills by using soap and water.
Oil-based paints contain natural oils like linseed, Tung, or soybean, or synthetic alkyds. They contain pigment, oil resin binder, and a solvent thinner. When spread on a surface, the thinner evaporates, leaving the resin and pigment to form a durable, long-lasting hard coat.
Oil-based paints often produce more VOCs, so they are less environmentally friendly, and surfaces are more difficult to clean. Oil-based paints work well on doors, trim and baseboards that require a more durable paint than most walls due to contact with vacuums, furniture, and grubby hands.
The paints take longer to dry but create a tough, durable, hard film. Cleanup is more difficult and requires paint thinners, turpentine, or mineral spirits to clean brushes, tools, and spills.
Liquids or Solvents and Additives or Fillers
Water in latex paints and mineral spirits in oil-based paints carry and spread the pigment color and binders onto surfaces, and then evaporate. Paints have different additives depending on their use.
- Fillers, such as clay, talc, lime, or barite, help thicken the paint film.
- Bonding additives improve the ‘sticking’ ability of paints to different surfaces.
- Enamel is added to paints to make them harder, more durable, and less porous when dry.
- Conditioners or anti-microbial additives improve the flow and reduce brush drag, resist mildew, and improve the leveling ability of the paint.
- Texture additives like fine sand to make stair runners less slippery.
- Glitter to enhance reflective properties.
- Scents are also added to make the paint smell better.
Pigments provide the color for paint and may be organic or inorganic. The type of pigment can also determine how durable the color is, which impacts how often the walls may need sprucing up or repainting. Organic pigments are either vegetable or animal in origin and are naturally transparent.
They traditionally aren’t as long-lasting as inorganic pigments, but modern chemistry is changing that. Inorganic pigments are solid mineral colors that occur naturally but are modified for paint manufacture.
Inorganic pigments are long-lasting and have been found on 30,000-year-old cave paintings. All white pigments come from inorganic sources, with titanium dioxide being the most common. Organic pigments mix, blend, and disperse in the base and have better transparency than the finely ground inorganic particles which are mixed and suspended in the base. A higher inorganic particle count creates more solid content, which results in deep rich colors.
Binders hold the pigment together to create a durable film on a surface. The plastic-like polymers provide better adhesion to different surfaces, and resistance to abrasion, and household cleaners. In latex paint, binders should be 100% acrylic, not a blend. Oil paints commonly contain drying oils such as tung, linseed, or modified oils.
Can You Use Interior Paint Outside?
There are many reasons for wanting to use interior paint outside. It may be less expensive, you want to use up extra paint, or you like the color, are just some reasons I’ve heard. However, while you can use interior paint outside, you shouldn’t.
Interior paints have a different formulation than exterior paints. They aren’t designed for freeze-thaw, rain, snow, mold, and mildew, and will chalk, fade, and break down more quickly. Interior paints are more porous, lack the necessary resin to bind them to exterior surfaces, and are more likely to crack and flake.
If you are determined to use interior paint outside, it should be sealed with one or two coats of exterior-grade, clear-coat sealer. Depending on the amount of exposure to the elements, it may extend the length of time it will adhere, but it won’t necessarily prevent it from fading, and the sealer may yellow.
Using interior paint with a sealer increases the amount of work, and makes more expensive exterior paints a cheaper finish. Additionally, using interior paint outdoors voids its warranty.
Painting the inside surface of a front door with interior grade paint is fine since the surface is protected from the elements. The surface will need to be properly prepared, especially if it’s a metal door. You can use the same paint on the exterior of the door if it is protected with a storm or glass door; otherwise, it may fade, crack, and chip.
Using interior paint on interior concrete walls or slabs is possible with a lot of prep work – filling cracks, TSP to clean the surface, a sealer, a masonry primer, and several coats of paint. However, using interior paint on exterior concrete requires an additional coat or two of quality exterior sealer over the interior paint. Exposure to the sun and other elements will still cause interior paints to fade and wear more quickly than exterior paints.
Some people wonder about using interior paint as an exterior primer. Whether to minimize cost or some other reason, you can use it as a primer. It will need to be lightly scuffed with sandpaper to improve adhesion and then covered with a quality exterior paint of the same base.
Unfortunately, you may still have to repaint the surface in a year or two depending on adhesive qualities and your climate extremes. Modern interior grade paints are formulated for interior use, not outside.
What Happens If You Accidentally Used Interior Paint Outside?
Whether you grabbed the wrong can of paint or just weren’t aware there was a difference until it was too late, accidents happen. Using interior paint outside isn’t the end of the world. It just means you’ll likely be repainting the surface in two to five years instead of ten to fifteen.
Interior paints are formulated for the protected surfaces within a building, not to withstand the abuses nature can toss around. It will fade more quickly and doesn’t have the additives to protect against temperature extremes, UV, or mildew. You can paint over it now with exterior paint, or wait and do it in a year or two.
If you’ve almost completed the job, you might as well finish it. If it’s just begun, then you can switch to exterior paint and paint over it. Some suggest painting over the interior paint with an expensive sealer and considering it a lesson learned. Alternatively, save money and buy a quality exterior paint in the same color and base instead of a more expensive sealer, and just apply another layer.
Can I Paint Over Exterior Paint With Interior Paint?
Interior paint and exterior paint are not the same. Modern technology and chemistry have improved the products for indoor or outdoor use. If you wish to apply an interior paint over an exterior paint, the quick answer is yes; the long answer is…why?
Applying interior paint over an exterior one requires surface prep work and the use of a primer if it’s water-based over oil-based paint. If the paint is exposed to the elements, it is important to note it won’t withstand temperature changes, humidity, UV, mildew, or weather factors as well as exterior paints.
Using interior paint over exterior paint may save some money initially. It won’t last as long, though, so you’ll be applying another coat sooner, which eats up any savings. It also increases the labor factor. If a renovation or addition has changed an exterior wall to an interior wall, you can confidently paint over exterior paint with interior paint. However, interior paint is not recommended for exterior use and commonly voids any warranty.
Interior paint is formulated for use inside and is best suited for that use. The paint can be used outside, but it won’t stand up well to weather extremes, and you’ll need to repaint sooner than planned.
I hope you have a better understanding of interior paint, why it is called interior, and what happens if it is used outdoors.
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