Wood stains achieve the same result whether they are applied to exterior decks or indoor flooring and furniture. They add color and vibrancy to aging wood. But there are some crucial differences between these products, so they are not to be used interchangeably. Exterior deck stain products often incorporate additional chemicals, such as fungicides, that may be unsafe for interior use. They are formulated for better durability, but produce fumes that simply aren't appropriate for an indoor environment.
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The presence of fungicides in exterior wood stains is perhaps the most important difference between indoor and outdoor stains. Exterior wooden decks endure additional environmental stresses. Because of their exposure to the elements, exterior deck stains almost always contain fungicides that kill mold, mildew and fungi. These chemicals can be toxic to humans, so the products should not be used on indoor wood floors or furniture. According to Green American, zinc oxide is the least toxic type of fungicide found in exterior stains, so it is perhaps the safest to use.
Deck stains incorporate preservatives and pesticides that indoor stains lack. They are often formulated to protect from UV damage, water damage and weathering. Indoor stains may also offer these protections, but to a lesser degree. Deck stains may exhibit richer pigmentation, which protects the sun-exposed wood from fading. The risk of chemical exposure to humans from fungicides makes exterior formulas unsafe for using in your home. But indoor stains are still toxic and both kinds contain some amount of volatile organic compounds, or VOCs.
Both interior and exterior wood stains come in oil-based formulas, which adds shine and durability to wood. Both types of stains are made to penetrate, rather than coat, the wood. They add color by absorbing directly into the wood. The wood can be gradually darkened with additional coatings of stain. Both deck stains and indoor stains are made to repel water and resist scuffing. A top clear coat is usually necessary to retain the staining and protect the wood.
Paints, wood stains and other finishing products, particularly oil-based formulas, are known to contain harmful chemicals. Never inhale the vapors of a wood stain before it is completely dry, even if it is an indoor stain. You should never use an exterior wood stain indoors if the manufacturer states not to do so. If you're not sure, review the ingredients and recommended applications. Contact the manufacturer to determine if the product is safe for your intended use.