1) Is there any misapplication in the following sentence?
His wealth today is more due to having guanxi up his sleeve than due to having the know-how to make money.
Guanxi (Guan-xi) is a Chinese noun phrase.
If you have no idea of guanxi, you might take a look at
Guanxi - Wikipedia, the free encyclopedia
In order to keep my skill from rust, I practice it from time to time.
In order to keep my skill from rustiness, I practice it from time to time.
I do not know which word is the correct use.