Why does the gypsum board ceiling turn yellow (Part 3)?

I have recently come across three cases of yellowing of gypsum boards. Brother Long’s experience is that yellowing of suspended ceilings is mostly caused by improper installation operations in the later stages. I have also written two articles to summarize and analyze them before (Why do suspended ceilings with paper-faced gypsum board partitions turn yellow? Why do suspended ceilings with paper-faced gypsum board partitions turn yellow (Part 2)?).

Today I will talk about how to correctly understand the problem of yellowing of gypsum board. I hope it will be helpful to you.

1. Reasons:

The primary ingredient in putty powder used in renovations is quicklime (CaO). When workers apply putty, especially when rushing to finish the job and the paint hasn't dried completely, a hydrolysis reaction occurs: CaO + H₂O = Ca(OH)₂. OH- ions form on the putty surface, chemically forming a weak base. The mildew inhibitor in latex paint contains a large amount of Fe₂+ ions, which are oxidized by air to Fe₃+ ions, resulting in the reaction Fe₃+OH- = Fe(OH)₃↓.

Fe(OH)3 is a brown, flocculent colloidal precipitate. When it appears on the surface of putty, it appears yellow. The pH value plays a key role in determining whether yellowing occurs.

Therefore, it's recommended to wait until the putty and paint are completely dry before reapplying the second coat to prevent OH- from dissociating. Furthermore, since bleaching agents may be present in both latex paint (coating) and putty powder, oxidation reactions with the oxides in the curing agent in the latex paint (coating) can easily produce a yellow product. Furthermore, latex paint contains a complex set of additives, which can deteriorate over long-term storage, leading to yellowing. An excessively high pH value in the putty powder is a major contributing factor.

Since the 0H- ions of putty powder react with the Fe3 ions of latex paint to form the yellowing substance Fe(0H)3, why is the putty on the wall not yellow but the putty on the gypsum board turns yellow?

Because, given the same surface area, the wall is many times thicker than the plasterboard, and the wall's volume is many times larger than the plasterboard's. Therefore, when the painter applies the paint evenly, the specific moisture content of the plasterboard is greater than that of the wall surface, causing it to dry more slowly, leading to the release of OH- ions in the putty powder. However, this creates the illusion of rapid drying. Gypsum board absorbs water like a sponge, quickly absorbing the water sprayed onto its surface. However, if the paint is not stirred thoroughly before adding water, the solids remain on the surface of the plasterboard's putty powder, where they oxidize to a yellowish color in the air. This is similar to laundry detergent that does not fully dissolve in water: after the water is absorbed, it will remain on the surface of clothes, causing them to turn yellow over time.

2. Prevention: Strictly follow the construction requirements

1) Before painting the walls, treat the drywall with moisture-proofing treatment, such as spraying a moisture-proof primer or wallpaper base film. Allow the surface to dry completely, usually 2-3 days before painting (this may take a while, but it's worth it to avoid any problems).

2) Follow the wall paint's instructions for application, which will specify the water-to-water ratio and re-spray interval. These instructions must be followed strictly. Adding too much water or re-spraying too quickly can cause a chemical reaction between the water and the plasterboard, leading to quality changes.

3. Solution

1) Don’t be impatient during construction to avoid color leakage due to the putty not being completely dry;

2) Purchase high-quality paint from genuine brands and legitimate channels;

3) Re-prime the yellowed areas and re-apply latex paint.

On the one hand, the alkali-resistant primer can increase the fullness of the topcoat film and improve the adhesion of the topcoat. On the other hand, it can effectively seal the alkalinity of the water-resistant putty to avoid the alkali back and blooming of the topcoat.