Why is my stone coat paint turning white?

Two days ago, a long-time customer called me: "Jason, our end-user's newly painted stone-coated wall has developed streaks of white after drying, like a layer of frost. The workers said there's a problem with the material. Could you please report this to your product department and see what's going on?"

I asked him to send me some photos of the site. Seeing the mottled wall, I almost immediately understood—it wasn't that the material was "bad," but rather that the user had encountered a problem during the application process. Our technical team immediately held a discussion meeting, and considering the characteristics of the acrylic emulsion we provide, we identified several possible causes. We communicated with the customer, eliminating these possible causes of the whitening one by one.

Although we are a supplier of coating emulsions (especially for water based acrylic emulsion supplier), one of the most widely used industries is architectural coatings. Having worked in this industry for twenty years, I've seen far too many similar "whitening incidents." Customers always assume it's a product problem, but the truth often lies hidden in easily overlooked details. Today, I will use this client's case to explore the three main reasons for whitening in stone coatings and how we can address the problem at its root.

I: Is your wall surface "clean"?

Stone paint is like a seasoned artist, demanding high standards from the canvas. If the substrate isn't prepared, it will protest with whitening.

The two main reasons are: the wall is too wet, or the wall surface is too alkaline.

We have conducted tests. When the substrate's moisture content exceeds 10% and the pH value exceeds 10 (i.e., too alkaline), the moisture carries alkaline substances to the surface, forming a white, frosty layer on the paint film. This is what we commonly call "alkaline efflorescence." The acrylic emulsion we provide has good alkali resistance, but if the substrate problem is too severe, even the best emulsion cannot withstand the continuous erosion of alkali.

A photo sent by the client shows a whitening area that looks like solid powder with blurred boundaries—a typical characteristic of alkaline precipitation. I asked, "Did your user apply a primer?"

After verifying with the client and end-user, my client sighed, "His client company is rushing to meet a deadline. The workers said the primer was too thin, it doesn't matter whether they apply it or not..."

Therefore, primer is crucial. It's like applying base coat to your nails—it isolates the alkaline substances under the substrate while allowing the subsequent "topcoat" (stone-textured paint) to adhere better. Skipping this step can cause many problems in subsequent construction. A high-quality sealing primer can form a good match with our acrylic emulsion, preventing alkali seepage from the source.

II. Inclement Weather: Did the client check the windows during construction? If so, were the windows intact during the construction process?

Besides insufficient substrate strength, weather conditions during construction are also a major "influencing factor."

The acrylic emulsion of stone-textured paint needs a certain temperature to form a film—just like milk forms a thin film after cooling, wall paint emulsion also needs a suitable temperature to form a complete protective layer. When the temperature is below 5℃ or the relative humidity is above 85%, the paint film cannot form. Moisture gets trapped beneath the paint film and eventually seeps out, causing a whitening effect after drying.

I remember a client in South Korea encountered the same problem. The temperature plummeted, but the project was on a tight schedule, so the workers continued working despite the difficulties. The next day, the entire wall looked like it was covered in a veil. We adjusted the construction time, allowing the temperature to rise around noon, and added a suitable film-forming aid (specifically formulated for our acrylic emulsion), which solved the problem.

For stone coating application, my advice is: ideally, choose a sunny or rainless day with a temperature between 10-25℃ and humidity below 80%. If the project schedule is indeed tight, a film-forming aid must be added to help the emulsion (acrylic emulsion) form a film under unfavorable conditions.

Here are some common water-based coating formulations for wall coatings.

How to Make: Thoroughly combine water, titanium dioxide, heavy calcium carbonate, mica powder, dispersant, and co-solvent. Grind this mixture to form a uniform color paste. Separately, using high-speed stirring, mix the emulsion with the remaining additives. Finally, incorporate the color paste into the emulsion blend, stir until homogenous, and then filter the final product.

III. Hidden Dangers in the Formulation: Excessive Additives are Harmful

We encountered another case where the problem wasn't at the construction site, but in the formulation.

To improve performance, stone coatings require various additives, including surfactants, to facilitate application. However, everything should be done in moderation—excessive surfactants, like too much MSG in soup, can be counterproductive. These excess additives can remain on the paint surface, absorbing moisture and causing white stains. This is especially true in systems where acrylic emulsions are the primary film-forming substance, where the compatibility between the additives and the emulsion is crucial.

When the problem is caused by excessive additives, we usually recommend reducing the amount of additives used or switching to a low-foaming acrylic emulsion—our technical team can assist in selecting a more suitable emulsion model.

Recently, we collaborated with a paint manufacturer to improve their formulation. We tested the whitening of stone coatings batch by batch. After testing both the paint and the substrate, we found no whitening. Ultimately, we discovered that the root cause was a batch of raw materials with excessively high additive content, which interfered with the film-forming process of the acrylic emulsion. After adjusting the formulation, product stability improved significantly, and customer complaints decreased markedly.

In short: whitening is not the end, but the starting point for solving the problem.

What happened to the customer we mentioned at the beginning?

We advised the customer to send technical personnel to the end-user's construction site. They used specialized instruments to measure the substrate's moisture content and pH value, confirming that the alkaline precipitation was caused by incorrect primer application. The solution was simple: scrape off the whitish areas, re-treat the substrate, apply a high-quality sealing primer, and then re-spray a new paint formulated with our acrylic emulsion as a base. A week later, the client sent photos—the wall was even and full, with no trace of whitishness.

In this case, because the client communicated with us promptly after encountering the problem, our professional team held an immediate meeting to discuss the issue. Considering the characteristics of acrylic emulsion, we quickly identified and investigated various possible causes of the whitishness.

As a supplier of acrylic emulsion, our greatest satisfaction isn't just about selling our products well, but also about being a trustworthy "technical partner" to our clients when they encounter problems, helping them find solutions.

FAQ

1. Q: Why are water-based architectural coatings becoming increasingly popular in the current market?

A: This is mainly due to increasingly stringent environmental regulations and increased public awareness of environmental protection. Water-based coatings use water as the primary carrier (solvent content ≤10-15%), offering advantages such as low VOC emissions, no irritating odor, and safe use, making them an effective measure to reduce air pollution and construction safety risks.

2. Q: What role does acrylic emulsion play in water-based coating formulations?

A: Acrylic emulsion is the film-forming resin of the coating and a key factor affecting coating performance. It also improves the coating's hardness, adhesion, water resistance, and weather resistance. Due to its excellent antioxidant and UV resistance properties, acrylic emulsion is widely used in interior and exterior wall coatings.

3. Q: What is the main purpose of adding fillers such as mica powder to the formulation?

A: Fillers not only help control production costs but also significantly affect coating performance. For example, mica powder can enhance film strength, heat resistance, rust prevention, and reduce permeability. In some special coatings, such as road marking paint, adding a sufficient amount of mica powder (18-25%) can significantly improve the coating's weather resistance.