Tips for fixing a leak in your radiator include removing the radiator from the car, identifying the radiator type, determining the location of the leak, sealing the leak with a radiant filler and patching the area with a weld epoxy. Replacing old solder with a new one may also help.
Start by draining the radiator, detach hoses, and remove the radiator taking care not to damage fins. Radiator coolant may flow sideways or vertically. Make sure to assess the tank to determine if it is made of plastic or metal, as well as the fins and tubes, to know if they are aluminum, brass or another metal. Stress joints and areas along seams between the tank and radiator are vulnerable to leaking.
If the radiator is made of plastic, pour some radiator filler into the tank, and let the filler run through to seal cracks. To apply a cold weld epoxy, clean off the site, put the port of a vacuum hose over the overflow line, and pry off the distributor cap while turning the motor to suck the epoxy into the radiator. For a radiator that is made of brass or copper, heat and sand the old solder, and apply new solder. Be sure to clean the damaged radiator seam properly before applying fresh solder.