Radweld is designed to be added directly into the engine’s expansion 
tank. You don’t need to drain or flush the system before you use it – just pour and go. Radweld works through the radiator channels to seal leaks without any clogging or damaging effects, so all you need to do is pour it in and let it repair the leak.
When adding Radweld, it is important to ensure the engine has cooled before unscrewing the cap of the expansion tank as the coolant operates at around 90°C. After filling with Radweld, make sure that the cap of the expansion tank is replaced and tightened. .
This video shows you how to use Radweld and what happens in your radiator after you’ve used it. You can also read more about Radweld by clicking the links below.