〉 Answered on Jan 4th, 2013 by Jo An Holt, Automotive Journalist at Holt & Associates
The simplle answer is, no. Coolant additives are meant to seal small cracks in a car's radiator. They may work for a brief amount of time in a car with a blown head gasket, but will soon leak again due to the extreme pressures bleeding from the cylinders into the water system.
〉 Answered on Jan 4th, 2013 by Karen Salvaggio, Owner - Professional Race Car Driver at Thunder Valley Racing
The short answer is no, they do not. Head gaskets typically consist of materials like copper, steel, graphite and polymer coatings. When a head gasket fails it is due to deterioration of these components which creates a sealing void that no liquid sealant can replace.