To repair an aluminum radiator with epoxy, turn the engine off, allow it to cool down and take out the radiator. Find the leak, and clean the area around it thoroughly. Take the amount of epoxy needed to fix the leak, and knead it in your fingers until it becomes soft. Apply it to the leak, and allow the material to dry overnight.
Make sure to clean the area around the leak or crack you are repairing of all the dirt and grease with a good cleaner. Rinse the area with water, and allow it to dry completely. If there is any dirt or moisture on the surface, the epoxy is not going to hold well on it. Be extremely careful when working with epoxy, as it is very toxic. Keep children and animals away from the working site. Make sure to use enough epoxy to apply on the leak. Remember that it is going to be under heavy pressure when the vehicle is running.
After the epoxy is dry, make sure that it holds firmly on the surface before reinstalling the radiator. Put the radiator in place, and start the engine. Slowly increase its power while constantly looking for leaks. If the radiator is still fine after the engine has been running for five to ten minutes, the repair is good. Thoroughly clean the epoxy off your hands and all the surfaces, and dispose of garbage properly.