Copper coat is for coating the bottom of boats to stop the barnacles growing, it's not a gasket compound.
Copaslip is a high copper content grease for anti seize, not a gasket compound either but it has been found when coated on copper head gaskets will help the gasket seal better. As does silver spray paint and a few other similar items coated on gaskets. https://www.molyslip.co.uk/copaslip/
So not applicable to a gasketless joint, stick with what Leon suggests, a good clean flat surface on both components.