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Old 04-13-2009, 10:21 AM  
bobw77
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It could have peeled because the people who sold the house used interior paint instead of exterior which is more resistant to weather and can't handle expanding and contracting. It just sounds like they wanted to top it off with something to make it look better but didn't want to buy outdoor paint. Use a wire brush to clean the brick of old paint and dirt and make sure w/e you use is outdoor otherwise you will be in the same boat 7 months down the road. GL.



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Old 04-13-2009, 07:23 PM  
Nestor_Kelebay
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Originally Posted by bobw77 View Post
It could have peeled because the people who sold the house used interior paint instead of exterior which is more resistant to weather and can't handle expanding and contracting.
BOB: I'm wanting to explain the difference between interior and exterior latex and alkyd paints because some of what you said is mostly true for latex paints, and the rest is mostly true for oil based paints. Explaining it will prevent people from assuming what you said is generally true for both kinds of paints.

Interior and exterior LATEX paints will both stand up well to wind and rain. In fact, Sherwin Williams uses exactly the same acrylic binder resin from Rohm & Haas (called "HG95P") to make BOTH their top-of-the-line interior latex paint and their top-of-the-line exterior latex paint (Super Paint I think it's called.) Exterior latex paints do better outdoors only because they have UV blockers in them to protect them from the UV light from the Sun, and they have more biocides in them to prevent mold and mildew growing on them in humid climates. Basically, its those extra additives (UV blockers and more biocides) that makes the difference between interior and exterior latex paints.

But, also:

1. Interior latex paints are often made with resins that dry to a harder film only because interior paints have to stand up to scrubbing to remove stubborn marks on the walls, but people rarely scrub the exterior of their house to clean it, and so exterior latex paints will often use acrylic resins that dry to a softer film to save cost.

2. Also, exterior latex and oil based paints will often use zinc oxide instead of titanium dioxide as the white pigment. This is both because titanium dioxide acts as a catalyst in the chaulking of the paint film due to exposure to UV light, and because zinc oxide is a natural mildewcide. So, all else being equal, white paints that use titanium dioxide as the white pigment won't stand up to UV light as well as white paints that use zinc oxide as the white pigment. Also, mildew and mold won't grow as well on white paints that use zinc oxide as the white pigment as they will on white paints that use titanium dioxide as the white pigment. But, zinc oxide doesn't hide as well as titanium dioxide, so you may have to apply a second coat of white or off-white paint if the white pigment is zinc oxide to completely hide any underlying colour.

3. Finally, interior latex paints will often use different coalescing solvents that either smell less or get chemically bonded right into the paint film as it dries, and so don't evaporate from the paint film as it dries, and therefore don't smell at all. It's also true that more and stronger coalescing solvents are needed in exterior latex paints that often have to form a proper film under more difficult conditions than interior latex paints, like cooler temperatures, higher humidity and more rapidly changing temperatures and humidities (come night fall or rain).

Now about oil based paint:

What you're saying is "true-er" with oil based paints. Alkyd paints form harder films than latex paints, and hardness and elasticity are kinda mutually exclusive. Interior alkyd paints dry so hard that they don't have the elasticity needed to stretch and shrink with wood as it's moisture content changes. Consequently, "exterior" alkyd paint resins are made from vegetable oils (the oil "lipids" actually)that won't crosslink as densely and will therefore dry to a softer and more elastic film. That is, exterior latex paints are intentionally made worse (weaker and softer, which you generally don't want in a protective coating) because you need elasticity in a paint that's meant to be used on wood outdoors (cuz it'll swell and shrink).

The few hundredths of a percent change in dimensions due to thermal expansion and contraction is tiny compared with the several percent diminsional change that occurs in wood. EVERY interior and exterior latex and alkyd paint has enough elasticity to accomodate thermal expansion and contraction in building materials. It's that several percent change in dimension in wood due to changes in it's moisture content that requires special "exterior alkyd paints" to accomodate.

Exterior oil based paints have more UV blockers and biocides in them than interior oil based paints too, just like latex paints. With latex paints, that's the primary difference between interior and exterior paints. With oil based paints, film hardness is the primary difference between interior and exterior paints.

Therefore, if you don't live in the south where UV light from the Sun is intense, or on the west side of a mountain range or along a coast where humidity is high and mildew growth on paint is a problem, then you can use an interior latex paint or interior oil based paint outdoors over any substrate other than wood. You can paint a steel garage door or concrete tire stops with interior oil based paint, for example.

In this case, the old paint was applied over concrete, which only has thermal expansion due to temperature changes which any paint can accomodate. So, the fact that the concrete woulda got warm in summer and cold in winter wouldn't have caused the paint on it to crack and peel prematurely.

That is, the fundamental difference between interior and exterior LATEX paints is the amount of additives (UV blockers and biocides) in the paint. The fundamental difference between interior and exterior OIL BASED paints is the hardness (and hence elasticity) of the film they dry to.

Both interior and exterior latex paints form soft enough films to accomodated the dimensional changes in wood outdoors.
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Old 04-15-2009, 10:01 AM  
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Hey, that old paint also could have peeled off due to poor surface prep and not moisture. An exposed concrete foundation, above grade, is not likely to have moisture issues unless something really weird is happening.

Regardless - The only proper course of action is to remove any and all loose paint, properly follow prep instructions and apply the correct paint as directed.

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