What's the Difference Between Interior & Exterior Paints
Different paints have different properties, and are
formulated for different purposes. Interior paint is made to be scrubbed, resist
staining, and allow cleaning. Exterior paints are made to combat against fading and
mildew. When starting a painting project, it’s important to know the differences between
the two and choose the right paint.
In order to understand the difference between exterior and interior paints, you will
need to learn a bit about the chemistry behind paint. Read on for an introduction to the
main components of all paints.
Some Basics of Paint
The components of all paints contain pigments, solvents, additives and
resins. In latex paint, the solvent is water, while in oil-based paint the solvent of choice is
mineral spirits. The solvent is what causes the paint to be ‘wet’. As paint dries, the solvent
will evaporate. You are left with the resins, pigments, and additives, which are the lasting
ingredients. The pigment is the color, which is bound to the surface by the resin. Resins can be
made of epoxy, acrylic or even silicone.
Additives give paint different properties, such
as resisting the growth of mildew, making the paint easier to apply, or even making it easier to
clean.
So What's Difference ?
While there can be many subtle differences, the primary difference
between interior and exterior paints is in their choice of resin. You will remember that the
resin is what binds the pigment to the surface. In an exterior paint, it is important that the
paint can survive temperature changes and being exposed to moisture. Exterior paint also must be
tougher and resist peeling, chipping, and fading from sunlight. For these reasons, the resins
used in binding exterior paints must be softer.
For interior paint where temperature is
not a problem, the binding resins are more rigid, which cuts down on scuffing and smearing.
Where Should you Use Over the Other ?
The added resins in exterior paint can cause outgassing, that may last
up to 48 hours, but usually continues in small amounts for years. This is one reason you
wouldn’t want to use exterior paint indoors. Choose a flat sheen exterior paint for masonry and
stucco, as this will allow the surfaces to breathe, and allow moisture to breathe through the
paint to escape. Brick walls especially should be allowed to breathe, as moisture transfer is to
be expected with brick.
Interior paints are delicate in comparison with exterior paints,
but don’t outgas in the same way, making them safer to use indoors. You should still make sure
there is adequate ventilation when painting indoors. For an indoor surface, you may need to
clean and scrub, choose a glossy finish interior paint, as they resist scrubbing better than
flat surfaces, which can smear.