Before applying flux, what preparatory step should you take concerning the pipe ends?

It's essential to prepare the pipe ends before applying flux to ensure a successful brazing process. Rounding the pipe end with an expander is crucial because it helps create a proper joint fit and allows for better flow of the molten filler material. A rounded end can help prevent gaps and create a more efficient joint, promoting better heat distribution during the brazing process.

While other options involve beneficial practices, they do not specifically address the preparatory needs for pipe ends before flux application. Removing surface contaminants is excellent for ensuring a clean surface for the flux to adhere to, while sizing the pipe is also an important step in ensuring compatibility with fittings. Adjusting the torch flame is critical for the actual brazing process, but it does not relate to the initial preparation of the pipe ends. Therefore, rounding the pipe end with an expander directly contributes to achieving a strong, effective joint, making it the most appropriate choice in this context.