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PaintingInterior vs. Exterior Paint: Why Mixing Them Up Is a Costly Mistake
They are both paint. They both go on a brush. So why can you not use them interchangeably? The answer lies in chemistry, and the consequences of getting it wrong range from annoying to genuinely hazardous.
by Luna Drywall and Paint
The Formulation Difference
Exterior paints contain flexible resins that allow the coating to expand and contract with temperature changes of 50 degrees or more between night and day. They also contain higher levels of mildewcides, UV-resistant pigments, and fungicides formulated to survive rain, sun, and biological attack. Interior paints prioritize low VOC output, washability, and touch-up characteristics.
What Happens When You Use Exterior Paint Inside
Exterior paints off-gas significantly more and longer than interior formulations. The mildewcides and biocides are not approved for continuous indoor exposure. Beyond health concerns, exterior paint in an interior environment dries too hard and brittle, making it prone to cracking and difficult to touch up cleanly.
What Happens When You Use Interior Paint Outside
Interior paint on exterior surfaces is primarily a durability failure. Without flexible resins and UV inhibitors, it will crack, chalk, and peel within one to two seasons, often sooner in the intense South Carolina summer sun. What seemed like a cost-saving shortcut ends up costing significantly more in prep and repainting costs.
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