4 Steps to Prevent Bubbles in Leveling Skim Coat

It is a huge headache for workers when small bubbles appear right after applying a leveling skim coat. Even worse, many people only notice these bubbles when they start sanding. Sanding a bubbly surface just creates more tiny holes and pits, leading to endless patching and wasted materials.

Most bubbling issues can actually be fixed before you ever start sanding. The key is to understand why they happen: the wall absorbs water too fast, or the skim coat traps air inside.

Here are 4 practical steps to solve the bubbling problem early:

1. Properly Seal the Substrate

Highly absorbent walls, like cement mortar or gypsum boards, draw water out of wet skim coat too quickly. This traps air and forms bubbles. Always apply a primer or bonding agent to seal the substrate first. This keeps the moisture where it belongs and allows trapped air to escape smoothly.

2. Use the Right Mixing Procedure

When mixing the skim coat powder with water, do not use it immediately. Let the wet mix sit undisturbed for 2 to 3 minutes to "slake" (mature). Then, mix it thoroughly a second time. This allows the chemical additives to dissolve completely and removes microscopic air bubbles caused by the first mixing.

3. Apply Thin Layers and Multi-Coats

Keep the thickness of each individual layer between 1mm and 2mm. If you apply the skim coat too thickly, air cannot escape and gets trapped inside. This issue becomes much worse during hot, dry, or windy weather.

4. Defoam While the Skim Coat is Wet

Right after scraping the skim coat onto the wall, use a specialized defoaming roller over the wet surface once. This instantly breaks deep-seated bubbles. It is the fastest and most efficient way to get a clean surface.

Summary

Bubbling is not a surface issue that you can simply grind away with sandpaper. By sealing the wall, applying thin coats, and defoaming while wet, you will get a perfectly flat, solid, and smooth substrate after sanding.