This is not mine, rather it's from Leeum of Xtremesystems.org which I think would be useful

Radiator repair guide

What you will need:

Soldering iron
Solder with flux (I find the best stuff is the 50/50 tin and lead)
Wet and dry sandpaper
Waterproof glue



The first task is to locate the leak, the most common place for damage and a leak to occur is on one of the tubes.

These tubes run between the rows of fins on the radiator. If you have trouble spotting the damage, fill the radiator with some water to see where it runs out.



Here’s the damaged area. As you can see there’s a small hole in the tube which has been pierced by a screw. This guide only really applies to small holes, if you’re dealing with something more severe then you’re going to have to consider brazing.



The next task is to get the surrounding area of the tube rubbed down with some wet and dry to remove the paint and see the extent of the damage.



Next get your soldering iron and slide it up the tube to the damaged area, get it nice and hot and then proceed with the soldering stage.

With a bit of luck you should be able to solder right over the top of the damaged area. Just to be sure, reach for your glue (in this case, waterproof angling glue) and coat the soldered area with it.



Then get some water flowing through the radiator for a good few hours and watch the repaired are for any sign of dripping water. If you do, rub down the surrounding area and repeat the process of soldering again.

If all goes well you shouldn’t see a drop come through