Outside or inside, what’s the difference with paint? Isn’t white paint at the end of the day just white paint with a bunch of different labels on it? If that’s the case, then there should be no issue painting an interior over an exterior paint.
In fact, the answer is no, paints are very different depending on which type and mix one chooses. Walk into a paint store or a big box hardware store for paints and anyone can end up a bit confused by the literal rainbow of selections.
Even in just the same color, one will find anywhere from five to ten different paint types, forms, versions, and application types. And that doesn’t even include the multiple paint brands available to choose from as well.
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Understanding What Makes Paint Different
The two big categories of exterior and interior paint tend to be the grouping that everything else falls under but, in reality, both work as a wall or surface paint. From this point, the paints then break down between water and oil-based.
The real questions then are, how long does a user want the paint to last and what kind of environmental conditions matter. The chemical makeup or composition of paint makes a big difference in its performance.
Some paints are simply constituted for color surfacing and absorption. Other paints are mixed to withstand the outside elements and last much longer under outside conditions as a result.
The General Nature of Exterior Paints
As noted earlier, exterior paints include chemicals and paints designed to last much longer due to outside elemental wear and tear.
Because the paint needed to be water-resistant to fend of the damage of rain and moisture, almost all exterior paints used to be oil-based. This has changed in recent years due to newer environmental restrictions, and most newer exterior paints are also water-based now as well.
The reason why exterior paint then lasts longer outside has to do with the binding resins. Given the outside conditions, the exterior paint has to deal with temperature changes, fading, and moisture.
All of these factors can break up the binding aspects of the paint mixture. So acrylic resins are used instead.
How Interior Paint is Different
Interior paint was never intended to be exposed to the elements outside. It is constituted to apply thinner and dry faster. This delicate nature of interior paint gives it an inherent weakness when used outside.
The second problem is absorption. Because of how it is constituted, normal exterior paint can be applied to a bare surface like cement or stone and will bond without issue. Standard interior paint needs a surface that has been prepared and etched even to bond correctly.
In many cases, a primer has to be used that dries with a slightly spongey consistency. This gives the final interior paint a surface to soak into a bond with when it dries. Some interior paints come with a primer component mixed in to make their application a one-step job.
The third issue is the lack of durability interior paint has to the elements. It is water-based, so rain and moisture will quickly eat away at interior paint as an outside layer. Due to the rigid resin chemical structure of interior paint, it breaks apart quickly.
And that will turn into peeling and flaking as well as fading in a very short amount of time in the sun. As soon as the rain hits, the flecks and bits wash off, and the general layer painted then begins to simply fall apart, exposing the under layer and primer.
Despite All the Drawbacks…
Yes, technically an interior paint can be applied on top of exterior paint, but it simply doesn’t make much sense. The absorption capability will be weak, most exterior paint is applied on outside surfaces where interior paint performs poorly, and the results after a few months will just look like flaking or peeling mess.
That’s a lot of effort and clean up work, later on, to get involved with for little in the way of results. On the other hand, if the exterior paint is on furniture or an inside wall, a new interior paint layer would work fine.
However, a primer layer always needs to be applied first to give the interior paint a proper “bed” to bind to and soak into. Without that preparation for the sublayer, the interior paint could end up peeling and ultimately failing as a new paint layer overall.