What tool can help spread the flux over the two surfaces after a joint has been assembled?

The tool that is most effective for spreading flux over two surfaces after a joint has been assembled is a brush. Brushes are designed to evenly coat surfaces with flux, ensuring that the filler material can flow properly during the soldering or brazing process. A well-coated joint promotes better adhesion and a stronger bond by allowing heat to transfer efficiently throughout the joint and maximizing the wetting action of the solder or filler metal.

Using a brush also allows for precision in application, which is important for areas where excessive flux could create contamination or make the surfaces unnecessarily messy. Other tools like a small twist, pencil, or knife may not offer the level of control or even application needed for effective flux distribution, thereby making a brush the most suitable choice for this task in soldering and brazing techniques.