4 Practical Ways to Reduce Bubbling in Gypsum Plaster

 Bubbling, blistering, and localized hollow areas are common problems encountered during gypsum plaster application.

When bubbles appear on the wall, the first reaction is often:

“Is there something wrong with the gypsum plaster?”

However, in many real construction projects, bubbling is not necessarily caused by the gypsum material itself.

In many cases, the actual causes are related to substrate absorption, substrate preparation, application thickness, and plastering techniques.

This is especially important when working on concrete walls, aerated concrete blocks, or brick substrates. If the substrate is too dry or absorbs water too quickly, the water in the gypsum slurry can be rapidly drawn into the substrate, making it difficult for trapped air to escape.

The result can be bubbling, blistering, or even hollow areas.

So, how can gypsum plaster bubbling be reduced?

Here are four practical steps worth paying attention to.

Step 1: Moisturize and Seal the Substrate — The Most Important Step

One of the most common reasons for gypsum plaster bubbling is a substrate that is too dry or absorbs water too quickly.

Highly porous substrates can rapidly absorb water from the gypsum slurry. This may lead to excessive water loss at the interface and make it easier for air to remain trapped between the plaster and substrate.

How should concrete walls be treated?

Before application, lightly moisten the concrete surface using a sprayer or roller.

The goal is not to completely soak the wall. The substrate should be moist but free from visible standing water.

If a bonding agent or wall primer is used, follow the manufacturer's application instructions and allow it to dry sufficiently and form a stable film before applying gypsum plaster.

What about aerated concrete blocks and brick walls?

These substrates are generally more absorbent and therefore require appropriate pre-wetting before plastering.

After wetting, there should be no obvious standing water on the surface. Allow the substrate to reach a suitable condition before application.

For highly absorbent substrates, a penetrating or sealing primer may also be considered depending on the project requirements.

The key principle is simple:

If the substrate is too dry, prepare it first. If it absorbs water too quickly, control the absorption rate.

Step 2: Apply the First Coat Thinly and Press It into the Substrate

To save time, some workers apply a relatively thick layer of gypsum plaster from the very beginning.

However, when the substrate has a high risk of bubbling, this approach may increase the problem.

For the first coat, a thin and well-compacted application is recommended.

A thickness of approximately 3–5 mm can be used as a practical reference, although the actual thickness should be adjusted according to substrate flatness, product characteristics, and project requirements.

During application, do not simply place the gypsum plaster onto the wall.

Use the trowel with appropriate pressure so that the material can properly contact and fill the pores and irregularities of the substrate.

This can help:

Improve contact between gypsum plaster and substrate;

Reduce interfacial voids;

Release some trapped air;

Reduce the risk of subsequent bubbling and hollow areas.

After the first coat, do not immediately apply a thick second layer. Allow the first coat to reach a suitable condition according to the site temperature, humidity, and setting characteristics of the material.

Step 3: When Applying the Next Layer, Press and Smooth the Surface in Time

After the first thin coat has been applied, the subsequent plastering process still requires attention to bubbling.

If small bubbles begin to appear during application, do not wait until the material has fully hardened before dealing with them.

While the material is still workable, use the trowel to press, compact, and smooth the surface.

When bubbles first begin to appear, timely pressure and smoothing can help release trapped air and reduce visible surface bubbles and pinholes.

A simple sequence to remember is:

Apply → Wait for the right condition → Observe → Press → Smooth

This simple action can make a significant difference in controlling bubbling on site.

Step 4: Control Air Entrapment Through Material Formulation

Substrate preparation and application techniques are important, but the gypsum formulation itself can also influence the final surface condition.

Factors such as wetting, workability, water retention, and air content in the slurry can all affect the final appearance of gypsum plaster.

Some construction sites may attempt to use small amounts of surfactants to improve wetting or reduce air entrapment.

However, it is important to note that:

Household detergents or dishwashing liquids should not be treated as standard additives for gypsum plaster.

Excessive addition may change the properties of the gypsum system and potentially affect setting time, strength, and adhesion.

If bubbling needs to be addressed at the formulation level, a more professional approach is to evaluate defoamers, cellulose ethers, retarders, and other functional additives through laboratory testing and optimize the formulation according to the specific gypsum system.

For projects with complex site conditions or highly absorbent substrates, a lightweight gypsum plaster specifically optimized for application performance may also be considered.

A Simple Rule to Remember

Moisten and prepare the substrate;

Apply the first coat thinly and compact it;

Press and smooth subsequent layers in time;

Treat bubbles while the material is still workable.

Gypsum plaster bubbling is rarely just a question of whether the material is “good” or “bad.”

Stable plastering performance depends on several factors working together:

Substrate Condition + Material Formulation + Application Technique + Site Environment

Only by considering all four factors together can the risk of bubbling, blistering, and hollow areas be effectively reduced.

If serious bubbling has already appeared on site, do not immediately replace the gypsum material.

First check the substrate moisture condition, water absorption rate, primer treatment, application thickness, and construction method. Then make targeted adjustments according to the actual cause.

Good materials provide the foundation. Correct application makes the difference.