Here's why:
* Adjectives describe nouns. "Rusty" describes the condition of something, like a tool, a car, or a skill.
* Adverbs modify verbs, adjectives, or other adverbs. "Rusty" doesn't describe how an action is done, so it's not an adverb.
Example:
* The rusty car wouldn't start. (Rusty describes the car)
* He had a rusty knowledge of French. (Rusty describes his knowledge)