Symptoms: You can't see your antifreeze, when you take off the cap (see: puddle under your car)
When your radiator is leaking, it's a better idea just to replace the entire thing, rather than trying to patch it up with welding or sealant. It's only delaying the inevitable. Start by jacking up the car and removing the drain bolt at the bottom of the radiator to remove all coolant. While that's going on, you can start removing the overflow tank, unplugging the cooling fans, removing the top radiator hose, and taking off the top brackets that help hold the radiator in place. If there's room, you can remove the fans right there, or you can wait to remove them after you pull it all out in one big package. If you have an automatic car, you'll also have to remove the transmission cooling line. From there, slide the new radiator in and swap all the tubes and rubber pieces. Finally, fill the car with either premixed fluid or a 50-50 mixture of antifreeze and water.