Clean, flat and smooth. These three words describe the surface in an engine, not just the head and block. But how clean, how flat and how smooth do the surfaces really have to be to get a good, long lasting seal? It depends on the application.
Clean means no dirt, oil, grease or residual gasket material on either mating surface. Both surfaces must be spotlessly clean and dry to assure a good cold seal.
One thing to watch out for here is the use of cleaning solvents or gasket remover chemicals that leave residue on the surface. The residue may interact with the coating on or materials in a head gasket leading to premature gasket failure. So be sure to wash any chemical residue off the surface after the parts have been cleaned.
Also, some rebuilders use an abrasive pad in an air drill or buffer to buff off residual gasket material on head, block and manifold surfaces. It