If your radiator is made of plastic and aluminum or just aluminum soldering is not gonna help. You can use the crimp and fold method on leaky tubes, but for any large cracks or holes in the tank you will need to try to use the epoxy or two-part epoxy putty. Clean and rough up the damaged area with a wire brush or sandpaper. The rougher the better so the epoxy has something to grab onto. Once you are done with any of these fixes you can reinstall the cap, cover one outlet with your hand, and use your mouth to lightly pressurize your radiator and listen for leaks or spray with soapy water or window cleaner. You may have lots of small leaks. If so now is the time to mess with them some more or test out that trick of adding pepper or some egg white and hope that fills the pinholes.