Proper repair depends upon where the leak is.  Coolant leaks often occur at the water pump shaft seal.  Other leak points include radiator, heater cores, hoses, freeze plugs, and the engine itself.

Small radiator leaks, as well as minor internal seepage inside the engine or past a freeze plug, can often be temporarily plugged with a can of sealer.  No sealer will plug a leaky water pump, hose, or badly damaged or corroded radiator.  The only cure is to replace the faulty component.

Small radiator leaks can often be patched by soldering or using a special high-temperature epoxy.  Other options include sending the radiator to a specialty shop for repair or recoring, or replacing the entire radiator itself with a new one.  With a leaky heater core, replacement is the best option.

Internal coolant leaks are more of a challenge to fix because they require more expertise to diagnose and repair.

Coolant can seep past a head gasket or hairline cracks in the engine block or cylinder head and enter the combustion chamber or crankcase.

If not stopped, such a leak can cause major damage to piston rings, cylinders and bearings, as well as overheating due to coolant loss.

An internal leak may be indicated if the cooling system is losing coolant without obvious leaks.  The first item to check is the radiator cap.  A weak cap that can't hold pressure will allow the system to boil over.  Both the cap and radiator should be pressure checked with a pressure tester.

If the cooling system fails to hold pressure, coolant is leaking internally.  The question is where?  A compression check on the engine will tell if a head gasket is leaking or if there's a hairline crack in the cylinder or head.

A higher than normal dipstick level and/or excessive moisture in the crankcase would indicate a coolant leak into the crankcase.  In either case, major engine work will be necessary to repair the leak.

Another type of internal leak is one that can occur between the coolant and automatic transmission fluid (ATF).  On most vehicles with automatic transmissions, ATF is routed through an oil cooler inside the radiator.

If the tubing leaks, coolant can enter the transmission lines, contaminate the fluid and ruin the transmission.  Red or brown drops of oil in the coolant would be a symptom of such a leak.

Because the oil cooler is inside the radiator, the radiator must be replaced to eliminate the problem.  Transmission fluid should also be changed.