It's caused by concrete interface:
1. low adhesion on the smooth concrete interface
2. the air covered by the mortar can't release into the wall
3. the bubbles will get more when you level more times.
4. the generally way to avoid this is to make the surface rough or use primer.
1. low adhesion on the smooth concrete interface
2. the air covered by the mortar can't release into the wall
3. the bubbles will get more when you level more times.
4. the generally way to avoid this is to make the surface rough or use primer.
