Rusty radiator: what causes it & how to fix

A rusty radiator is the result of a chemical reaction between the metal of the radiator, and oxygen in water droplets on the radiator’s surface. Oxygen reacts with the iron to form a iron oxide, a reddish-brown substance which we know as rust.

When we think of radiators and rust, we tend to think of internal corrosion and the build-up of sludge inside them. This problem typically results in radiators which are cold at the bottom. However, without the right care, your radiators can also go rusty on the outside.

Why do radiators go rusty at the bottom?

Water vapour in the air condenses onto the radiator and forms water droplets. As the droplets grow, they become heavy enough to run down to the bottom of the radiator. Small cracks in the paint allow the water to come into contact with the metal, allowing the chemical reaction to take place.

Rusty radiator in the bathroom

Chances are, you’re reading this page because your rusty radiator is in the bathroom. And that’s no surprise due to the steam when people take a bath or a shower. However, it could happen in any room with high levels of humidity.

Not only do rusty radiators look bad, they can also cause serious problems. Over time, the radiator will start to develop ‘pinhole’ leaks. Such leaks start off small and can be very difficult to accurately locate. You may not even notice them at first, but given enough time, the rusted area can disintegrate completely and cause a flood.

How to prevent rusty radiators

This problem is a classic case of “a stitch in time saves nine”. The best thing you can do is to stop the problem before it occurs. First of all, inspect radiators for signs of flaking or absent paint. Cheap radiators may show signs of poor workmanship – use a mirror to inspect the edges underneath, and paint over any signs of exposed steel. You will need a special radiator paint for this, and don’t paint over existing rust, as this will simply hide the problem without actually fixing it.

If you’re buying a new radiator or having a new one installed, it’s likely that you can paint it even if it looks like it’s been painted already. That’s because, on many radiators, the white layer of paint is actually a primer that is designed to be painted over. A proper layer of paint will help to ensure that your radiator stays rust-free for many years. Check the manufacturer’s instructions to see if this is possible.

How to fix a rusty radiator

If the rust on a radiator is superficial, it may be possible to remove it by sanding it down and repainting it.

First of all, turn the radiator off and let it cool down. Then, prepare the area where you will be working so you don’t get paint where you don’t want it. You can put newspaper on the walls with masking tape to protect them, and on the floor to catch debris. Make sure the area is well-ventilated. Wear a suitable mask so that you don’t inhale airborne particles of paint and metal. Clean the radiator with a soapy cloth to remove any dirt on the surface.

Use sandpaper to remove the paint and rust from the relevant spot, and expose the steel of the radiator itself. Take a look at the edges of the area that you are sanding down. Some areas of rust may be hidden under the existing paint, and there may be more than you think.

Clean the area with a damp cloth and then dry it. Once it is dry, paint it with a suitable radiator paint. You may need to apply more than one coat.

Once you have finished, give at least 24 hours for the paint to dry.

Replacing it altogether

In some cases, there can be so much rust that it would take more effort than it’s worth to sand it all away. Alternatively, the rust could be so bad that any attempt to sand it away could trigger a leak. Or, the rust is in an awkward place where the only way to remove it is to drain the radiator down and remove it from the wall.

In these cases, it may be a much better use of time and money to replace the radiator.