WordNet records no synonym and no antonym for rustiness. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the condition of being coated or clogged with rust
ineptitude or awkwardness as a consequence of age or lack of practice
“his rustiness showed when he was asked to speed up”
Synonyms, antonyms and the broader and narrower relations for rustiness come from Princeton University’s WordNet. Full sources and licences.