One of my steam radiators has a small leak under the bottom. No hole is visible, but water drips out at a rate of a few drops per minute. Various websites suggest applying a 1 mm thick layer of J-B Weld after sanding and cleaning with acetone.

Does the fact that the radiator leaks necessarily mean that it is completely rusted from the inside and that leaks will soon appear elsewhere? Or is it likely that a slow leak is not due to rust, or that the rust is in only one place?

The radiator is decades old (perhaps even a century), but has been moved recently in order to repair the floor. The leak is clearly away from the valve.

  • Follow-up: The J-B Weld repair worked fine, but a second leak soon opened up in another location. Commented Jan 1, 2013 at 10:54

4 Answers 4


I haven't had very good luck with repairing steam radiator patches from the outside (sample size: 2). Steam under pressure is very good at finding microscopic holes between patch and substrate, and enlarging them.

What I have done successfully is an inside patch with epoxy, using a wire brush and steel wool to prep the surface. Getting at the leak may be a challenge, particularly with a radiator that has more than 4 sections you may need to break the sections apart (if you do, this is an excellent time to get the exterior sandblasted and painted).

Simplest and most cost-effective solution is probably to find an architectural salvage shop and replace the radiator (they might take your old radiator as a partial trade).


Be sure that the leak is actually in the radiator itself, and not where the valves meet the radiator body: water is very good at travelling a good distance horizontally thanks to surface tension. I check this with a bit of paper towel.

If the radiator is newly installed it may well be that the hole will sort itself out by filling with mineral deposits; if it's a hot water based system there's also proprietary sealing gunk you can add to the water that'll seal small holes.

If it's not new... then yes, I fear it's rusted.

  • Thanks. The radiator has been there for decades (or maybe even a century), but has been moved recently when repairs were made to the floor. I should add that to the question. Commented Jul 21, 2010 at 19:33
  • I fear if it's away from the valve that it may well be rusted through. Commented Jul 21, 2010 at 19:38

If the radiator has started to go, it may be a futile exercise to plug leaks. Nevertheless, several pointers are:

  1. Make sure you know for a fact exactly where the leak is by using flour or talc, eg. Steam is under pressure and can spray into a fine mist through a tiny hole that collects on a surface and runs down. The drop of water might appear far away from where the leak is actually occurring.

  2. If corrosion is involved you should clean it out before making the filling, like a dentist.

  3. Metal of two different types will expand and contract at different rates. This tends to make metal "patches" of any kind fail after time if the patch materials is not EXACTLY the same as the host material.

  4. One trick that can be used in instances where the leak is small and circular is to use a screw. You drill a hole where the leak used to be. Then you tap the hole, ideally with a fine machine thread. Finally, insert a machine screw, perhaps with a single layer of teflon tape. If done right, it will perfectly seal a hole.

  5. If the leak is shaped like a fissure, it is very difficult to fix. For a cast iron shell the only real possibility is a thermite weld, but it requires a lot of expertise and experience to do this right, so it is rarely cost justified unless the cast iron thingy is VERY valuable or irreplaceable (not the case here).


There are several products made to seal leaks in car radiators. I have seen some amazing results from these, even stopping a "gusher". If the metal has corroded away from the inside, since they are usually metal based, they might even replace lost materials from the deepest spots.

  • Please update your post with a link to one of these products. Thanks!
    – Niall C.
    Commented Dec 28, 2014 at 17:52

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