A car's radiator performs an indispensible function, helping the engine to maintain its proper temperature by forcing coolant through the engine and the rest of the cooling system. The radiator also radiates heat away from the vehicle through its surface. Whether a radiator is made of traditional brass or the newer plastic materials, a leak in the seam is one of the worst leaks that can occur in a radiator, because the coolant will spew from it quickly and the radiator must be removed in order to be repaired.
Run the engine until the car is warm and the coolant is circulating through the radiator and engine. Observe the radiator for spewing, dripping or seeping coolant and the location of leaks. Clear any debris from the radiator's surface with a water hose or pressurised air hose. Dirt and leaves may have attached themselves to the radiator, obscuring the leak, but the size and location of the seam leak will determine the difficulty of the repair.
Allow the engine to cool and drain the radiator completely. Remove the radiator hoses and disconnect the radiator from the vehicle, unscrewing the radiator braces and disconnecting the radiator hoses. Sand the area of the leaky seam in preparation for welding.
Solder the broken or cracked seam and hold together until the solder cools and hardens.
Wave the air-free plastic welder over the broken or torn seam until it heats up. Place the plastic welding rod over the seam and melt into the break or tear with the air-free plastic welder. Hold together until the welding rod cools.
Squeeze the tube of epoxy into the seam and spread evenly. Press the seam closed and hold until the epoxy begins to harden. Allow time for the cold weld process to set.
Replace the radiator once the welding process has had time to set. Make certain to connect the hoses tightly. Refill the radiator with coolant, then start the engine to allow the coolant to circulate through the cooling system.
While the above steps should provide a stable repair of seam leaks, the radiator should still be taken to a professional to make certain that the repair was done correctly.