Clockwork Dragon Rust Breath

I’m GMing an AP that includes a Clockwork Dragon with the Rust Breath variant.

I cannot find a single resource that indicates what the Breath weapon actually does, so I’m asking you how you’d rule it. Treat it like Rusting Grasp? Rust Monster? Something else? Your input is appreciated.

Rust Breath (CR + 0): Rust-breath clockwork dragons do not breathe fire. Instead, the creatures breathe out a fine mist of an alchemical solvent that instantly rusts exposed metals. This so-called “rust breath” issues forth in a 60-foot line. Creatures can attempt a DC 22 Reflex save to avoid the effect for attended items; however, they need to make a separate attempt for each exposed item. Each alchemical dragon can hold enough of the alchemical solvent to make up to three breath weapon attacks before the solvent must be refilled manually, which takes 10 minutes. These clockwork dragons are made of ironwood, adamantine, and other resistant materials immune to rusting of any form.

Rust Breath (CR + 0): Rust-breath clockwork dragons do not breathe fire. Instead, the creatures breathe out a fine mist of an alchemical solvent that instantly rusts exposed metals. This so-called “rust breath” issues forth in a 60-foot line. Creatures can attempt a DC 22 Reflex save to avoid the effect for attended items; however, they need to make a separate attempt for each exposed item. Each alchemical dragon can hold enough of the alchemical solvent to make up to three breath weapon attacks before the solvent must be refilled manually, which takes 10 minutes. These clockwork dragons are made of ironwood, adamantine, and other resistant materials immune to rusting of any form.

Okay so now we know it rusts things. What effect does that have, mechanically?

Well you can do it 1 of 2 ways.

Against a metal creature do the "fire" damage appropriate for the creature.

Against metal objects, save or become broken save against a second breath weapon or be destroyed.

Or it does 14d6 damage but only to rustable objects.

I think I'm going to run with the second of Cavall's approaches. Breaking their stuff in the final dungeon of the AP sounds harrowing and adds some fear to the encounter.

Sounds fair. I more based it on rust abilities anyways. I would still do the "breath" damage to metal creatures. Not..uh... that that's likely to come up.