Final answer:
Excess flux from a soldered copper tube joint should be removed using a damp rag. This ensures the joint is clean and prevents potential corrosion that could be caused by residual flux.
Step-by-step explanation:
When soldering a copper tube joint, it is important to remove excess flux after the soldering process to prevent corrosion and ensure the longevity of the joint. Flux is an acid that facilitates the flow of solder but can be corrosive if left on the metal surface. The correct way to remove excess flux is using a damp rag. A damp rag will effectively wipe away the residual flux without leaving any moisture that might cause additional corrosion or without spreading oils that might contaminate the joint. An oily rag would be inappropriate as it could introduce contaminants, while a dry rag might not efficiently remove all the flux.