Bubbles in varnish can be caused by different factors. To avoid these bubbles, it is important to consider the following points: 

1. Humidity: If the air is humid when you apply the varnish, wait for a dryer day or heat the room before starting the work. 
2. Temperature: Avoid applying varnish when the temperature is too high. 
3. Tools: Use a good quality brush for even application. Ardec recommends either the Smooth Finish, Perfect Finish or Brilliant Finish brushes by Nour for best results.
4. Avoid shaking the varnish too vigorously before applying and let it rest for at least 15 minutes. 
5. Make sure to leave enough time between each coat to allow the varnish to dry properly. 
6. Clean the surface well before applying varnish, as dirt and dust can also cause bubbles. 

If you encounter bubbles, you can try lightly sanding the surface and applying an additional coat of varnish.