How to Solder a Radiatorby Derek Odom
If the damage to a radiator is not extensive, a repair can many times be done through soldering. Soldering is not like brazing or welding, as it does not physically bond two metals together; it is more like a patch, but a pretty good one. As with welding, more is not necessarily better. As long as the solder job is a quality one, the fix could last for the lifetime of the radiator.
Prepare the damaged area for soldering. This includes removing any paint with a small grinding wheel or wire brush, along with removing any burrs or sharp edges that may exist. If using a grinder, be very careful not to worsen the damage, because radiators are made from very soft metal. Then clean the area with a strong solvent such as Simple Green or Brake Cleaner so that the solder adheres to the radiator without problems. While preparing the area, keep your eye out for other damage, as well.
Solder the damaged area completely. Use a high-wattage soldering iron and very thick soldering wire. Heat the gun thoroughly so that the solder melts well and bonds with the surface of the radiator. Heat the damaged portion of the radiator with the soldering iron and introduce the wire slowly, trying not to drip solder into the radiator itself. Slowly work your way down the crack until it is completely covered with solder. Remember to unplug the iron when you are done soldering, because most do not turn off by themselves.
Grind the solder job so that there are no burrs or sharp edges. This can be done with a small wheel or wire brush. The idea is to make the solder line as smooth as possible and remove any contaminants. The repaired portion of the radiator can also be repainted or left alone. Fill the radiator with coolant and let the vehicle reach operating temperature while keeping your eye on the soldered area for leaks.