Bar's makes several head gasket repair sealers. The best head gasket sealer Bar's leak makes is the HG-1 Head Gasket Sealer. It is the strongest head gasket sealer and it is antifreeze compatible as well. No need to drain the antifreeze to use this head gasket sealer. Here are the instructions.
Make sure your engine is cold.
Remove the radiator cap only if the engine is cold. If the engine is hot or you don't have a radiator cap, remove the cap from the overflow reservoir.
Shake the head gasket sealer container.
Add Bar's head gasket sealer to the radiator. If there is no radiator cap, you can add it to the overflow tank. For fast results, you should add the head gasket sealer to the radiator whenever possible.
Top off the radiator with coolant.
Start the engine.
Set heat to maximum using the climate control buttons on the dashboard.
Keep the engine RPMs at 1000 for at least 15 minutes.
Turn off the engine and allow it to cool down for at least one hour.
How much Bar's head gasket leak to use?
Cooling system under 10 quarts / 2.5 gallons: Use 1/2 bottle (.5L).
Cooling systems over 10 quarts up to 20 quarts (5 gallons): Use 1 bottle / 1 L.
Next, fill the cooling system with water. Don't top off the radiator as you will need to add the BlueDevil head gasket sealer latter.
Ensure the engine is cold and remove the radiator cap.
Start the engine.
Set heat to max. temperature and blowing fans to max. using the controls on the dashboard.
While the engine is running on idle, start adding BlueDevil via the radiator filler hole.
Reinstall the radiator cap. Let the engine run for 50 minutes. During this time it is important to monitor the engine temperature. Don't let your car to overheat. If the temperature gauge goes into the red, you should turn off the engine let it cool for a bit and restart it.
After the engine has run for at least 50 minutes, let the engine cool down then reinstall the thermostat.
Drain half of the water from the cooling system. Fill the rest of the cooling system with Full Strength Antifreeze. Don't use 50/50 antifreeze.