The top coat brush can harden if it comes into contact with the Activator.

You can prevent this from happening in future:

• Always observe the drying times of 2 minutes between Activator and Top Coat.
• Before you apply the top coat, you can dab the surface dry again with a piece of paper towel.
  
  If your brush is already hard, you can simply swap it for the Brush Saver and leave it to work overnight. The brush will be as good as new the next day.
  
  Before you swap the brushes, please make sure to dry both well so that the two liquids do not mix.