Will a radiator stop leak additive really stop a coolant leak?

Yes, but only if the leak is in a component that normally responds to such a product.

Leaks that can often be sealed with an additive include radiator and heater core pinholes (but not cracks), seepage leaks around engine freeze plugs, thermostat, manifold and head gaskets, and porosity leaks in aluminum heads or blocks.

Leaks that do not usually respond to additives include leaks in hoses, the water pump (around the shaft), or the radiator cap. Large cracks, cracks along outside corners or where hoses attach to the end tanks on the radiator are also very difficult to seal.

Even if a leak is sealed by an additive, no manufacturer will guarantee that such a seal will last. Repairing or replacing the leaky component, therefore, is the best cure -- and the only one that will guarantee lasting results.