Hair-kicking is common
New World terrestrials often develop bald patches after repeatedly kicking urticating hairs. The exposed skin can look pale at first and darker later.
Premoult darkening
A previously pale bald patch may become dark and glossy as a moult approaches. That colour change is one of the classic premoult signs in suitable species.
Not every bald patch is damage
Normal rubbing against a retreat or substrate can also thin hairs. A clean patch without injury is different from an open wound.
Do not apply products
There is no need for creams, oils or sprays on an ordinary bald patch. The hairs are renewed at the next moult.
Reduce unnecessary disturbance
If the spider is constantly kicking hairs, review how often the enclosure is opened or the retreat is disturbed.
When to worry
Look for bleeding, a ruptured abdomen, dark damaged tissue or other injury signs. Baldness alone is usually cosmetic and temporary.