ineptness vs rustiness

ineptness and rustiness both are nouns.

Word	Noun	Adjective	Verb	Adverb
ineptness	Yes	No	No	No
rustiness	Yes	No	No	No

As nouns, rustiness is a hyponym of ineptness; that is, rustiness is a word with a more specific, narrower meaning than ineptness:

ineptness: unskillfulness resulting from a lack of training
rustiness: ineptitude or awkwardness as a consequence of age or lack of practice

Other hyponyms of ineptness include ham-fistedness, ham-handedness, heavy-handedness.

ineptness (noun)	rustiness (noun)
the quality of having the wrong properties for a specific purpose	ineptitude or awkwardness as a consequence of age or lack of practice
unskillfulness resulting from a lack of training	the condition of being coated or clogged with rust