What should be applied to a cleaned tube surface before joining?

Before joining two cleaned tube surfaces, it's essential to apply a thin film of flux. Flux serves multiple crucial functions in the soldering and brazing processes. Firstly, it promotes wetting, which improves the ability of the molten solder or filler material to flow and adhere properly to the surfaces being joined. A good bond is vital for ensuring the integrity and longevity of the joint.

Additionally, flux helps to prevent oxidation of the metal surfaces during heating. Metals can oxidize quickly when exposed to high temperatures, and without flux, this oxidation can create a barrier that inhibits proper metal-to-metal contact, ultimately compromising the joint's strength.

Using other substances such as oil or paint can be harmful, as they may interfere with the joining process. Oil can create a layer that prevents adequate adhesion, while paint can burn off or break down under heat, also leading to poor bonding. Water, while it may assist in cleaning, does not provide any benefits as a surface treatment before joining. Thus, applying a thin film of flux is the proper procedure in preparation for soldering or brazing.