A lot of the auto parts store shelf head gasket/block repair stuff will require you to fully drain and flush the cooling system before you put it in. Then you put it in and do the treatment and then you have to do a complete drain and flush and refill again before you go back to coolant. I.e. most cannot be mixed with coolant. It becomes quite an ordeal to get it done according to the label. But some you just dump into the radiator, and that’s probably the kind of thing you’re after. So if you try one of those things then pay attention to that.