John,
You can apply PPP by paintbrush. You can thin the PPP to a thin slurry, with water & apply with a paintbrush - ideal for the joints in a wing root. If you apply the putty & allow to dry, you can remove a lot of the surplus, using a stiffer paintbrush & water. Wet the dried putty, and use the stiff brush to scrub the surplus away, it takes a few minutes for it to work, but once it starts........................
You do have to be careful not to take too much off, and allow it to dry before final sanding ( also don't thin it too much! ). PPP is never going to be as hard as other fillers, but it's easy and quick to use. Remember to clean the cap & firmly seal the tube - it can dry out very quickly.
Dave