Quick answer: rust only ruins garden metal when the layer keeps flaking off and exposing fresh metal. On cast iron and thick steel the first oxide skin stays bonded, blocks most of the oxygen underneath, and slows corrosion to a crawl. Two things predict the outcome: wall thickness, and whether water can pool somewhere it never dries.
Does rust eat through garden metal, or just sit on it?
Both, and the same material does either one depending on conditions. Iron oxide takes up more volume than the iron it replaces — roughly twice the volume for the dense dry forms, and past six times for the hydrated rust that actually forms outdoors. That expansion is the entire mechanism. If the layer stays attached, it just becomes a tight crust that sheds rain. If it detaches, the expansion pries it off, fresh metal meets air, and the cycle restarts.
What decides which way it goes?
Wall thickness. This matters more than anything else. Atmospheric corrosion of plain carbon steel runs about 0.01 to 0.025 mm a year in a rural setting and passes 0.05 mm a year within a few miles of salt water. A casting with a quarter inch of iron behind it has decades in hand. Stamped sheet measured in fractions of a millimeter does not.
Alloy. Cast iron carries roughly 2 to 4 percent carbon plus a little silicon, which changes how the oxide forms and makes it cling harder. Stainless works differently again: at about 10.5 percent chromium and above, a chromium oxide film forms instead of iron oxide, and it regrows after a scratch.
Where water sits. Rust needs liquid water plus oxygen, not just humid air. Anywhere water collects and stays — the inner lip of a planter, a joint where a stake meets a bracket, the base of a leg sitting in soil — corrodes several times faster than an open face.
What it stands in. Soil holds water against metal for weeks. Mulch holds less. Gravel and an open paving joint drain and dry. On a cast-iron wall planter, the 10.6-inch frame dries fully because air reaches both faces; the same piece half-buried in a bed would fail at the soil line first.
When is rust on garden metal actually a problem?
Four checks cover almost every case.
Rust bleeding onto the surface below. Orange staining across a patio or wall means the layer is loose and washing away in runoff. That is active corrosion, not patina.
Flaking you can lift with a fingernail. Patina is bonded. If a chip comes away and shows bright metal underneath, the cycle is still running.
Thin walls. Press a thumb against an unused section. Springs back, fine. Flexes or crunches, and thickness is already gone.
Rust at a weld or fastener. Joints are where thin material meets load. A bracket carrying an 8.7-inch hanging birdhouse puts the whole load through two small points. A rusted-through one is a safety issue rather than a cosmetic one.
Should you coat rusted garden metal to stop it?
Usually no, and coating worn rust-finish pieces often backfires. Clear lacquer traps moisture underneath, clouds within a season, and then flakes off carrying oxide with it. If you genuinely want to arrest corrosion on bare metal, the order that works is: wire-brush away loose material, apply a rust converter so remaining oxide becomes a stable phosphate layer, then paint. Paint needs a sound surface to bond to; sealing over loose rust seals nothing.
For pieces meant to keep weathering, the useful maintenance is drainage and drying rather than coating. Clear the drain holes on planters and plant holders each fall, keep the base of stakes out of standing water, and let air reach the joints.
What is the short version?
Rust is the finish on most garden iron, and it is doing its job while it stays in place. Watch for flaking, staining, and trapped water instead — those three show up years before anything structural moves.