We have been using UltraCopper on copper head gaskets for years with zero coolant leaks.O-ring and reciever grooves with copper gaskets is a foolproof way of sealing compression,but unless you get 100% annealing of the o-ring in the gasket and 100% gasket enbeddment into the reciever grooves the flat surfaces tend to not seal.Apply a coating of UltraCopper silicone around all coolant and oil passages on both the block and head side,being carefull not to put it to close to the oil passages so it doesn't migrate into the passages causing restriction.It is important to snug down the head bolts and allow the silicone to cure for at least 24 hours,then final torque the bolts.It works better than any of the coated copper gaskets.Sometimes the copper gaskets are too hard and must be heated to get proper annealing and some gaskets are stamped from a sheet of copper and the thickness varies.Soft evenly rolled gaskets are the best and ones that are lasered or CNC cut are more uniform in thickness.
If your block and,or heads are o-ringed or have reciever grooves and you want to use Cometics then fill the grooves with a filler like Alvins' LabMetal or similar.