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Using an exterior paint for interior applications is absolutely wrong in all cases. You risk triggering allergic reactions and other medical ailments when using exterior paints indoors! Never, never use exterior paints indoors!

The most important idea to understand is that exterior paints contain mildewcides and fungicides, chemicals which destroy mildew and fungi that can destroy paints. Just like pesticides (which kill pests, insects, spiders, etc.), there are indoor and outdoor formulations based on need and strength. Paint companies are regulated to produce indoor paints which are safe for habitation. However, because exterior paints are not intended for use indoors, they can contain chemical formulations which trigger allergic responses. While these responses may not be life-threatening, over the course of time constant exposure to unintended chemicals can have unpredictable medical effects.

If that isn't enough to deter you, consider the durability question. Some people assert that exterior paints are more durable, yet cost the same as interior paints. While that may be true for a set of circumstances, the fact that outdoor paints contain harsh chemicals should be enough to stop their use indoors.

Consider the room that is said to receive the most benefit from exterior paints, the bathroom. What characteristics should bathroom paint have? It should be humidity and stain resistant, have good adhesion and contain mildewcide/fungicide (in the appropriate strengths). But interior paints should also be very smooth and enjoyable to the sense of touch - many exterior paints do not have this function, because hands touching the house exterior is a less frequent event than hands touching bathroom walls. Also, the humidity resistance of exterior paints is different than bathrooms. Bathroom humidity is generated by a warm bath/shower causing water vapor to fill the air - warm vapor. Exterior paints are designed to deal with exterior precipitation - rain, snow, sleet, ice - which is primarily cold. While exterior paints may serve the purpose of interior paints for a while, they will not exhibit the durability of paints intended for the purpose selected.

Here is another example. Exterior paints are applied to siding, brick and metal. Interior paints are applied to drywall and wood. Each material has a special characteristic concerning paint adhesion, durability and finished appearance. Drywall is a very porous substance, and while new drywall should always be primed to prevent poor appearance, interior paint may have built-in capabilities for handling this material.

The overall principle - it's an outdoor paint for a reason.

Learn more about this author, J. W. Nusz.
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