What should be applied to a solder joint after cleaning it?

After cleaning a solder joint, flux should be applied to ensure a strong bond between the solder and the metal surfaces being joined. Flux serves multiple important purposes in the soldering process; it helps to remove oxidation that may have formed on the surfaces, promotes wetting of the solder to the metal, and improves the overall flow of the solder during heating. By applying flux, you facilitate a better connection, which is crucial for the stability and longevity of the joint. This step is essential in preventing cold solder joints that might lead to leaks or failures.

In contrast, painter's tape, sealants, and waterproof adhesives do not provide the necessary properties for soldering. Painter's tape is not designed for high-temperature applications and would likely burn or degrade when heat is applied. Sealants are used for different purposes, such as filling gaps or sealing joints after installation, but they do not aid in the soldering process itself. Similarly, waterproof adhesive is not relevant to soldering as it is not intended for metal bonding at high temperatures. Thus, applying flux is the correct practice to ensure a successful solder joint.