G4 Polyurethane Primer is used to prime concrete, metal and brick surfaces to ensure re-inforcement matting and bandaging can be correctly applied. Without the use of G4 primer the re-inforcement matting and bandaging will not bond correctly. This will leave an area exposed to the risk of water damage. G4 primer must still be wet when materials are laid over it to ensure a fully waterproof seal is achieved.