Silver solder is the next step up from cold weld. Silver solder uses heat from a commercial soldering iron to melt flux and solder together. This creates a bond that you cannot get from off-the-shelf cold weld products. The application of silver solder requires more preparation, but it will last longer and hold better than other types of radiator leak repair. To properly apply silver solder, the radiator must be clean and dry.
Remove the radiator cap. Slide the bucket under the radiator and open the valve on the bottom of the radiator. Completely drain the radiator.
Scrub the crack that's the source of the leak with the wire brush. Sand the crack until the metal around the crack is shiny. Clean the area thoroughly with the cloth.
Apply the flux compound along the length of the crack. Let the flux dry for 15 minutes.
Apply the hot soldering iron to the flux. When the flux begins to melt, apply the solder to the area of flux that is melting. Allow the solder to melt and blend into the flux. When the melted area is covered with solder, move the soldering iron along the crack, melting and applying solder until there is melted solder the length of the crack. Wait 30 minutes for the solder to dry.
Refill the radiator with the coolant from the bucket. Put the radiator cap back on.
Find a flux applicator that is in a pencil-like tube if possible. These handy applicators make it easy to apply flux directly to the crack.
Do not over-sand or you could make the crack worse. Just sand lightly until the metal is shiny.