Interior paint on exterior siding...

My hired help primed my new hardboard siding with latex exterior primer, but painted the house with interior latex paint. The screw up happened at the store, the attendant picked the wrong paint from the shelf and nobody caught it until two weeks after the house was painted.... Anyway, what do I do now? Will a top coat of exterior latex solve the problem?

A top coat of exterior latex paint would solve the problem completely, if there was one, but most likely there is no problem here to solve. I realize you painted with a latex primer and paint, but the next paragraph talks about oil based paints just to explain why, when using oil based paints, the difference in the hardness of the paint is critically important.

There are separate "Interior" and "Exterior" oil based paints is because oil based paints form a much harder and stronger film than latex paints. The problem is that wood outdoors will swell and shrink considerably with changes in it's moisture content caused by seasonal changes in temperature and humidity. Interior oil based paints simply don't have the elasticity needed to accomodate those large dimensional changes in wood outdoors. Consequently, paint companies will (quite literally) RUIN interior oil based paints so that they don't crosslink completely, so that the resulting paint remains sufficiently soft, flexible and elastic to be used over wood outdoors. Such ruined paints are called "EXTERIOR" oil based paints. Really, the most important difference between an interior and exterior oil based paint is the hardness and rigidity of the film it dries to. Exterior oil based paints won't dry to as hard or rigid a film so that they don't crack and peel when used on a wooden fence outdoors.

Exterior oil based paints also contain MORE UV blockers and mildewcides that will prevent deterioration of the paint from exposure to UV light from the Sun and from mold and fungii growing on the paint in hot and humid conditions.

Interior and Exterior LATEX paints are a totally different kettle of fish. BOTH interior and exterior latex paints have more than enough elasticity to accomodate the dimensional changes in wood outdoors. Really, the only difference between interior and exterior latex paints is the amount of additives added to the paint. Exterior latex paints will have more UV blockers and fungicides in them to prevent the paint from deteriorating from intense sunlight or having fungii and mildew grow on the paint under warm, humid conditions.

Thus, while the difference between Interior and Exterior oil based paints is a critically important one, the difference between interior and exterior latex paints simply involves the amount of UV blocker and mildewcides in the paint. (The other difference is that INTERIOR latex paints will typically use titanium dioxide as the white hiding pigment, whereas exterior latex paints will typically use zinc oxide as the white hiding pigment.) This is because titanium dioxide acts as a catalyst in the reaction by which both oil based and latex paints "chaulk" (or deteriorate to form a powdery film) due to exposure to UV light from the Sun. Zinc oxide doesn't have that catalytic effect on the one hand, and the zinc metal in it is actually a natural biocide on the other that helps prevent fungii and mold growing on the paint in warm humid conditions. So, zinc oxide works well as a substitute for titanium dioxide in exterior paints.

But, you live in Minnesota, which has a climate similar to that of Winnipeg, Manitoba, where I live. You don't have intense enough sunlight to make either UV blockers or chaulking due to UV light from the Sun a problem. Also, you don't have the high humidity that would cause mildew to grow on paint outdoors like coastal cities like New York, Miami, Los Angeles and Seattle do.

So, in my humble opinion, the interior latex paints that was applied to your house will probably last just about as long as an exterior latex would have. That's because where you live, you really don't need the additional UV blockers or fungicides in your paint, nor do you need to be concerned about titanium dioxide promoting more rapid and extensive "chaulking" on your house's paint. It would be better to have used exterior latex paint, but the worst possible scenario here is that your interior paint won't last as long outdoors as an exterior paint might have. And, on top of that, the "problem", if there is one, can be entirely corrected by simply painting over that interior paint with an exterior paint.

So, if it wuz me, I would probably just try to make some agreement with the paint store to either give you a discount on the paint already purchased (since it might not last as long as it should). Maybe if they gave you 50 percent off or something like that, you'd be happy.

PS: (you don't need to read the rest of this post. It simply explains the difference between an "Interior" and "Exterior" latex primer, and it's not a big difference)

You stated that it was an "exterior" latex primer" that was applied to your house. Please be advised that in MOST cases a latex primer will be advertised as being suitable as both an interior and exterior latex primer. The only reason your primer was classified as an "EXTERIOR" latex primer is because the coalescing solvent in the primer smelled more than another latex primer that company makes.

Latex primers and paints are nothing more than a SLURRY of both clear, white and/or coloured particles (called "pigments") and clear hard plastic blobs (called the "binder resin" all suspended in a mixture of water and a low volatility water soluble solvent (called a "coalescing solvent" or "coalescing agent").

When you first apply a latex primer or paint to a wall, the first thing that happens is that the water evaporates, and that leaves each of those hard clear plastic binder resins surrounded in coalescing solvent at a steadily increasing concentration. The coalescing solvent then softens the hard clear plastic resins so that they get mushy and easily deformed. Then, the same forces (surface tension and capillary pressure) that cause tiny water droplets to coalesce into large rain drops in clowds take over and cause each of those soft mushy binder resins to pull on it's neighbors to cause those mushy plastic resins to form a continuous soft mushy film with no air spaces in it.

Then, the coalescing solvents evaporate from the soft mushy film causing that "newly painted" smell to fill the air in the room. As the coalescing solvent evaporates from the primer or paint film, the soft mushy plastic hardens back up again, leaving a relatively hard film of either Plexiglass or Polyvinyl Acetate on the wall with the clear, white and coloured pigment particles suspended inside it much like the raisins in raisin bread.

The above process is called "coalescence" is it's how ALL latex primers and paints form a film.

The fact that your primer was called an "exterior" latex primer just means that the coalescing solvent in it smelled. Many paints now use coaleasing solvents that either don't smell, or don't evaporate and become incorporated right into the primer or paint film. These "Low VOC" or "Zero VOC" latex primers and paints are odorless, so they're marketed as "Interior" latex primers or paints instead. The idea is that you'd want to use a low or zero VOC latex primer indoors so as to avoid any smell in the house. Only a few years ago, there was no such thing as an interior latex primer or an exterior latex primer as all of them were labeled as Interior/Exterior latex primers. Now, since we have coalescing solvents that either don't smell or don't evaporate from the paint film, there's a reason to call some latex primers interior or exterior depending on what coalescing solvent they use.

Let me know if you still have further questions and/or concerns about using an interior latex paint on the exterior of your house.

You can learn more than you need or want to know about latex paints at the Paint Quality Institute website at:

Painting information and resources for home interiors and exteriors - Paint Quality Institute

That web site was created by the Paint Quality Institute, which was established and funded by the Rohm & Haas Company, who were the largest manufacturers of the plastic we call "Plexiglas" in North America. Rohm and Haas produces acrylic resins used for the production of latex paints, acrylic floor finishes, acrylic grout sealers and acrylic nail polish as well as Plexiglas rods sheets and tubing. The Rohm & Haas company has been purchased by Dow Chemical, but Dow continues to fund the Paint Quality Institute and it's web site is still on the internet.