Patching cracked radiator
If the plastic is cracked there isn't much hope in patching it, the time you would spend taking it out cleaning the surfaces to be patched, applying the patch letting it dry reinstalling it refill the cooling system just find out that it didn't take you would be better off replacing the radiator.
If it is cracked in an area that is easy to get to and the crack is not large try some JB Weld and follow the instructions. It the crack is inaccessible, time for a professional radiator shop to look at it or buy a new radiator.
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