What brush on mattvarnish?

Two quick questions:

• what brush on matt varnish do you guys use? I use a very old one from vallejo, but I am not sure it still works perfectly, since it is 20 years old
• how do you apply it? I apply it in a thick layer, and clean up afterwards with a clean brush. However sometimes some bubbles are visible on the mini (see the spear of the farseer, honestly this is only visible on the photo but still worries me for the next model).