Final answer:
A mask during a microdermabrasion treatment is best applied with a spatula to ensure even and sanitary application, with the layer thickness dependent on the product's guidelines and desired effect.
Step-by-step explanation:
During a microdermabrasion treatment, a mask should ideally be applied using a spatula to ensure an even and sanitary application. Whether to use a thin or thick layer can depend on the specific product's instructions and the desired effect of the mask. Thick layers may be recommended for masks that are meant to deeply hydrate or nourish the skin, while thin layers might be suitable for lighter, quick-absorbing products. It's not typically necessary to use a sponge for application, but a sponge may be used to help remove the mask in a gentle and non-abrasive manner after the treatment.