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PROBLEM: When I apply paint to my home’s exterior stucco (currently white), the paint changes color. The grays change to purple. The whites change to yellow. Greenish-beige becomes orange. I have purchased many samples of paint, and a color change is apparent for all but the darkest colors. The change in color starts happening immediately when I apply the paint, and it becomes more intense over subsequent days. After a few days, there is a radical difference between the paint’s intended color and the new color.

Apparently a chemical reaction is occurring between the paint and a substance in/on the wall, but I don't know what it is, or how to remedy it.

SURFACE FACTORS:

  1. The house is stucco, so perhaps the surface is very alkaline. The house was built in 1971, and several coats of paint have been applied by the previous owners since then, so I would think the alkalinity would no longer be a factor, but perhaps it is. (Would it even be possible for improperly cured stucco to remain highly alkaline for 44 years?)
  2. The previous owner told me that, around fifteen years ago, he painted the house with "pool paint." (Unfortunately, I don’t know what type of “pool paint” was applied, or why it was applied.) My guess is that this was in response to the color-changing problem rather than the cause of the problem, but I don’t know for certain. (It is interesting that the pool paint has remained white all this time, while I’m unable to get a true white.)

I have painted the wood next to the stucco, and the paint on the wood appears exactly as it should and it remains constant(aside from becoming just slightly lighter as it dries). In other words, the paint only changes color on the stucco. Since the wood was never painted with pool paint, and since the wood is obviously not stucco, I still don’t know which factor (pool paint, stucco, or something else) is causing the color change. However, this does show me the paint color is truly changing on the stucco.

I have some chemistry background, but I'm really stumped by this problem.

Any help in figuring out what’s going on would be very much appreciated.

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  • $\begingroup$ A agree that a house cannot stay alkaline for decades given that we have rain to wash off most of the base and CO2 in the air to neutralize the rest. We don't know composition of "pool paint" or current paint (those are trade secrets). Besides this field is quite small, so I doubt we can answer it. Your best route is either to ask manufacturers of current paint or just double/triple paint the house. $\endgroup$
    – sixtytrees
    Commented Jul 20, 2016 at 0:18
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    $\begingroup$ Did you apply a sealer and primer prior to painting your topcoat? $\endgroup$
    – long
    Commented Feb 21, 2017 at 2:33

2 Answers 2

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You haven't told us the color of the stucco you are painting, nor the type of paint. Chemical reaction is very unlikely, paint that changes color would make for bad paint. Pigments are chosen to be stable. Old stucco would be only a little alkaline, but it is porous. This sounds more like color leaching out of the stucco and into the new paint. You need paint with better coverage (covers contrasting colors) or better sealing ability. Seriously, don't ask us, talk to your paint supplier at a paint store. Maybe get them to refer you to their in-house experts. You probably will have to spend more for the paint than you are planning on.

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Shouldn't be happening!

Neither of the stated special conditions have any apparent reason to cause this problem.

Stucco applied in 1971 will very probably be a mortar of Portland cement and sand. As you note, it is quite alkaline but this is perfectly normal and doesn't normally affect the colour of paint. The stucco is often applied over a layer of wire mesh to help it adhere to the wall -- the alkalinity actually inhibits corrosion in this mesh. If the mesh does corrode, this could certainly affect colour, but it is a gross defect showing obvious patterning. When it is well-cured, damp stucco can support a range of fungal and algal growth but again, these are fairly obvious in appearance.

Pool paint can be any of a wide range of paints, at least including acrylics, epoxies and chlorinated rubber. What they have in common is that they are designed to adhere well to Portland cement, and to provide a waterproof layer. Ironically this is likely to raise moisture levels inside your stucco as it will prevent damp from evaporating easily; but again there is no obvious reason this might affect colour.

Old paint exposed to the sun "weathers", and it can be hard work to strip this off a rough surface. However it tends to affect adhesion, rather than colour.

Approach systematically

I would recommend that you do some controlled tests to isolate the problem. Get some small sample pots of your preferred paint, and a couple of other types. Ideally find a patch where the existing paint is peeling. Apply small colour strips to: exposed stucco; peeled-off paint fragments, as collected; peeled-off paint fragments that have been carefully washed; to nearby Portland cement that is not part of the stucco; and to a "control" surface that is expected to show the colour well. Carefully record which strip is which. The test strips can be very small, so long as you can clearly identify the colour change.

This inexpensive test should help you to identify if the problem is associated to:

  • the stucco
  • subsequent paint layers
  • a contaminant on the exterior of the painted stucco
  • your paint batch or brand
  • the local environment generally

It may or may not help to identify the root cause, but could at least help to find a work-around.

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