TL;DR
Rust rings on a shower shelf almost never come from the caddy itself. They come from the metal bottoms of shaving cream cans and razor stands sitting in a permanent puddle. Clear soap scum with a mild acid, lift rust with an oxalic acid based rust remover, and then fix the cause: drainage and a dry contact point. A plastic or silicone coaster under a can ends the problem outright.
Two different problems on one shelf
Soap scum is the chalky gray film. It forms when the fatty acids in soap react with calcium and magnesium in hard water, producing an insoluble deposit that grips metal and plastic. It is mineral, which is why an acid removes it and detergent does not.
Rust is iron oxide, and it needs iron to exist. A stainless steel or aluminum caddy will not produce it in normal use. What produces it is the exposed steel rim on the bottom of an aerosol can, a razor with a steel component, a can of shaving cream, or a chip in a chrome-plated wire caddy that exposes the steel underneath.
Water sits at those contact points, oxygen is present, and rust forms and then stains everything it touches.
That is why the rings are always circular and always exactly where a can was sitting.
The same standing-water mechanism drives the slime described in how to clean a soap dish that stays slimy.
How do you clean a shower caddy?
Take it down, rinse it with hot water, and scrub with a brush and dish soap. Soak it for thirty minutes in one part white vinegar to two parts hot water to dissolve soap scum, scrub again, rinse thoroughly, and dry it completely before rehanging.
Clearing soap scum first
Rust removal works better on a clean surface, so do the scum first.
1. Take everything off and take the caddy down. Work in a sink or a tub where you can rinse freely.
2. Rinse with hot water. Heat softens the film.
3. Scrub with a brush and dish soap. A stiff brush for wire, a softer one for coated finishes. Get into corners and around welds.
4. Soak in a vinegar solution. One part white vinegar to two parts hot water, thirty minutes to an hour. Vinegar dissolves the mineral part of soap scum, which soap alone cannot touch.
5. Scrub again and rinse thoroughly.
6. Dry completely with a towel. This step matters more than any of the others.
For plastic and coated caddies, avoid abrasive powders, which scratch and give scum more grip. For stainless steel, work with the grain rather than in circles.
One absolute rule: never mix vinegar with bleach or with any chlorinated cleaner. That combination releases chlorine gas. Never mix bleach with ammonia, which is in some glass cleaners, or with rubbing alcohol. Use one product at a time, rinse it away completely, and ventilate the room.
Removing the rust rings
Once the surface is clean, deal with the orange marks.
Consumer rust removers. The rust removers sold for household use are typically oxalic acid based, and they dissolve iron oxide chemically. Follow the label exactly, wear gloves, ventilate, and rinse thoroughly. Test a hidden area first, since oxalic acid dulls some finishes and is not appropriate for natural stone. Never use a professional-grade industrial acid product at home -- some contain acids that cause severe injuries and are not consumer products.
A baking soda paste. For light surface rust, a thick paste left on for thirty minutes and scrubbed with a nylon brush lifts a surprising amount, with nothing hazardous involved.
A vinegar soak. Long soaks in undiluted white vinegar dissolve light rust over a few hours. Rinse and dry completely, since vinegar leaves metal prone to flash rusting if left damp.
Do not use chloride bleach on stainless steel. Chlorides pit stainless and make corrosion worse.
Do not use steel wool. It leaves iron particles that rust immediately, creating new orange specks.
If rust has eaten through a chrome plating on a wire caddy, the plating is gone and rust will keep returning at that spot. That is a replacement.
Why does my shower caddy rust?
Almost always because a steel-bottomed can or a razor is sitting in water on the shelf. The caddy itself may be stainless or aluminum, but the can rim is bare steel, and it rusts and stains everything under it. Put a coaster under cans, or store them out of the shower.
Preventing it from coming back
This is the part that actually solves the problem.
Put something between the can and the shelf. A small silicone or plastic coaster, a cut piece of a plastic lid, or a shelf liner. Break the metal-to-metal contact and the rust stops.
Store aerosol cans outside the shower. Shaving cream in particular. Bring it in when you need it.
Choose a caddy that drains. Open wire, slats, or a mesh bottom. A solid tray with a rim holds water permanently.
Hang it where the spray does not hit it directly. A caddy that gets rinsed with every shower never dries.
Shake the caddy and wipe it after showers. Ten seconds of drying prevents most of it.
Run the exhaust fan during and after showers. Fifteen to twenty minutes afterward makes a genuine difference across the whole room.
Wipe razors dry and store them out of the water. Rusting razors are both a stain source and a shaving problem, covered in how to clean a razor and razor handle.
The same lift-and-dry discipline is what keeps a mat mildew-free in how to clean a rubber bath mat and its suction cups, and it runs through cleaning the personal items you use every day.
Source: U.S. Environmental Protection Agency guidance on controlling moisture in the home
Choosing a caddy that lasts
Stainless steel. Good corrosion resistance, but grades vary and cheap stainless does spot in a wet bathroom. Look for a higher grade, and never use chloride bleach on it.
Aluminum. Does not rust, though it can develop a white oxide film that scrubs off.
Plastic and silicone. Immune to rust entirely. They collect soap scum like everything else, but scum is easy.
Chrome-plated steel. The cheapest and the most likely to fail. Once the plating chips, the steel underneath rusts permanently.
Bamboo and teak. Attractive, but they need drying and oiling and will not tolerate standing water.
Suction-mounted and tension-pole caddies both trap water at their mounting points, so check and dry those spots too.
FAQ
How do you get rust off a shower caddy?
Clean off the soap scum first with a vinegar soak, then use a consumer rust remover, which is typically oxalic acid based, following its label exactly with gloves and ventilation. A baking soda paste left thirty minutes handles light rust. Never use steel wool, which leaves iron particles that rust again.
Why does my shower shelf get rust rings?
Because a steel-bottomed can, usually shaving cream, is sitting in standing water on the shelf. The can rim rusts and stains everything under it, even if the caddy itself is stainless or plastic. A small silicone or plastic coaster under the can stops it completely.
How do you remove soap scum from a shower caddy?
Soak it for thirty minutes to an hour in one part white vinegar to two parts hot water, then scrub with a brush and rinse thoroughly. Soap scum is a mineral deposit formed by soap reacting with hard water, so an acid removes it while detergent alone does not.
Can you use bleach on a shower caddy?
Not on stainless steel, where chloride bleach causes pitting and makes corrosion worse. It is also unnecessary, since vinegar handles soap scum and a rust remover handles rust. Whatever you use, never mix bleach with vinegar, ammonia, or alcohol, and ventilate the room.
How often should you clean a shower caddy?
Rinse and wipe it weekly, and do a full vinegar soak monthly in hard water areas. Weekly attention keeps soap scum soft enough to rinse away, which avoids the long soak entirely. Dry the contact points where cans and razors sit every time.
What kind of shower caddy does not rust?
Plastic, silicone, and aluminum caddies do not rust at all, and good-grade stainless steel resists it well. Chrome-plated steel is the type most likely to fail, since rust starts wherever the plating chips. Whatever the material, drainage and drying matter more than the metal.
Caddies are a ten-minute job. A shower that is genuinely clean top to bottom is a bigger one. If you would rather hand that off, get a quote or read more about house cleaning services.