Final answer:
When retouching with a sodium hydroxide relaxer, apply the relaxer to new growth only to prevent overprocessing previously relaxed hair and potential hair damage.
Step-by-step explanation:
When retouching a client's hair with a sodium hydroxide relaxer, it's important to apply the relaxer to the new growth only. Sodium hydroxide is a strong alkali used in various hair relaxing products. Its function is to break down the disulfide bonds in hair, thus straightening it. However, due to its high pH, it can cause damage if misapplied or left on the hair for too long.
If applied to hair that has already been relaxed, sodium hydroxide can lead to overprocessing and breakage. Therefore, it should not be applied to the entire hair shaft, especially parts that have been previously relaxed. Moreover, using the relaxer for a shorter amount of time than during the initial application is also incorrect as the timing should be based on the texture and condition of the new growth, not on the previous application duration. Lastly, unless there is a specific reason, changing to a different type of hair relaxer is not the standard procedure for a retouch; consistency in the type of relaxer used can help prevent adverse reactions.