How can you repair a slight crack in your radiator?

barrs stop leak will work on most smaller holes and cracks if you can not find that you could try silver stopper also found at most part stores

If you can see the crack, you could use some fine sandpaper (when it's cool) and sand down the area to be patched, then wipe it clean, and apply some 'quicksteel' compound- found at most hardware or auto supply stores. JB weld works well, too. Ideally, since quicksteel sets fast, put this on first, wait an hour, then apply a thin coat of JB weld on top, and let it sit overnight before driving. You may even put some stop leak fluid for good measure.