What is the recommended method to protect the skin of an airplane during countersinking?

Using a nylon foot cage or protective tape is the recommended method to protect the skin of an airplane during countersinking because it provides a balance of flexibility and durability. When countersinking, the objective is to create a smooth, flush surface for fasteners without damaging the surrounding material. A nylon foot cage surrounds the area being worked on and absorbs vibrations, minimizing the risk of deformation or unintended scratches on the airplane's skin.

Additionally, protective tape can be applied to the area to further safeguard against scratches or dents that might occur from tools or debris. This method allows for precision in the countersinking process without compromising the integrity of the surrounding aircraft skin. The combination of both the nylon cage and tape effectively reduces the likelihood of damage while performing this critical task.

The other methods, while they may offer some level of protection, either lack versatility or may not conform as well to the delicate nature of aircraft materials, which can lead to potential damage or ineffective results. Thus, using a nylon foot cage along with protective tape ensures optimal protection during countersinking operations.