Can a leaking radiator be mended?

The simple answer I'm afraid is no, you will have to take the radiator off to seal the leak, there are brands of sealant that I have used successfully but you need to clean it properly etc. to get it to be secure when refixed to the system, and then it might spring a leak somewhere else on the rad.
the best thing is to close off the valves both sides, and with lots of old cloths around or idealy a wet and dry vacuum, undo the big nuts that secure the valves to the radiator, only a little bit at a time, (remember that the nut stays on the rad so when you look at the rad end on it looks like you are going backwards ie it undoes clockwise, if you look from the rad to the valve it undoes anticlockwise) slowly undo the nuts until water starts to come out, there will be roughly one pint per foot of rad, catch it (in a plastic ice cream tub is my favourite) and at any time you can do the nut back up to stop the flow of water, it will help the flow of water to start if you undo the bleed screw to let air into the rad.
once the water has stopped flowing and you can undo the valve and lift it away, the very first thing to do is to block the end of the stub that goes into the rad. a plastic bag with elastic bands, cork, anything, then you can undo the other valve and block that as well.
The reason is that there will still be left in the rad some sediment, this is black sludge that stains badly, so when you have blocked the ends and lifted the rad off of the brackets, take it ouside, remove the bung and tip it up on end to pour this horrible stuff out.
Reseal it and then take it to your nearest builders merchant so that they can identify it and give you the correct length, BTU output etc.
If its a very old one you might have to replace the brackets it hangs on but thats not tooo difficult if you think about it, the most important thing is to get it the same length so you dont have to alter the pipes
You will need to keep the stubs to fit the valves back on, they usually undo with a very large allen key inserted into them, you could buy new valves whch would have the stubs but they might not mate up with your valves, try being the little female and talk nicely to the builders merchant blokes asking them if they'll fit your old ones to the new rad.
then just take it home stick it on the wall do up the valve nuts tight and open the valves. JOB DONE!

Derek
June 2004