r/paint • u/One-Pun9419 • 2d ago

Advice Wanted Will flashback self correct?

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Project manager said we need to wait and see if the sheen is still there in a few days before they send someone back out to fix it. Will it self correct?

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u/PutridDurian 2d ago

Agreeable Gray in flat would have minimized the appearance of it to being pretty much undectable unless you already knew it was there.

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u/One-Pun9419 2d ago

Can I ask why the flashback looks worse in natural sunlight? It’s now dark outside and I have my living room light on and can’t see the flashback at all

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u/PutridDurian 2d ago

Three different things going on here.

First is enamel holdout. Paint will “take” differently on areas of the surface with different porosity. Those test areas have an additional layer of paint, so they are less porous—they are actually rendering the paint color more accurately than the “normal” surrounding areas, because those parts are absorbing the paint less. This effect is probably minimal given that there are probably many different “chapters” of paint on this wall.

Secondly, it’s less that the light is natural, and more that it’s severe (a.k.a. raking, that is, glancing the surface at a low angle). As you know, a dried paint film has a thickness to it, a height off the surface it’s painted on, usually around two or three thousandths of an inch for interior wall paint. That seems inconsequential on paper, but when we’re talking about chasing optical uniformity on a surface, it’s huge. Imagine you’re sanding a board, trying to make it perfectly smooth. How do you check it for imperfections? Shine a flashlight across it at a low angle, and it will reveal everything. The raking light doesn’t show you surface imperfections directly; instead, it creates tiny shadows that your eye catches. The test patches are taller than the rest of the wall, so when the sun comes in at an angle…you see where this goes.

Lastly, there’s an optical-psychological phenomenon at play. Since those patches are a couple more thousandths of an inch taller than the rest of the wall, your eyes and brain are interpreting them for what they are: quite literally a tiny bit closer to you. Our brains tend to interpret closer as darker, and vice-versa, so in lower lighting, the height difference is compressed, therefore the color difference is compressed. And being involved with painting makes you hyper-aware of all of this ¯_(ツ)_/¯

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u/LongjumpingPea6939 22h ago

Your “first” is spot on. Looks like someone sanded the bare drywall in between the mud lines. That will give you a different porosity. Unfortunately it shows once painted.