Final answer:
Flux should be applied to both the aluminum surface and the filler rod during oxyacetylene welding of aluminum to remove the oxide layer and ensure a quality weld.
Step-by-step explanation:
When oxyacetylene welding aluminum, the flux should be applied to both the surface of the aluminum and to the filler rod. Flux is crucial because aluminum oxide has a higher melting point than the aluminum itself. The flux removes the oxide, allowing the weld to form properly. It's important to use an appropriate type of flux specifically designed for aluminum to avoid contamination and to ensure a quality weld. Applying flux correctly is essential to prevent weld defects and to achieve a strong bond.