PURPOSE: Primer which can enhance adhesion between tetrafluoroethylene copolymer coat and a ceramic substrate, obtd. by incorporating a silane coupling agent, a supermicroparticle of anhydrous silica and water.

CONSTITUTION: Primer contg. (a) 0.5W10wt% silane coupling agent, (b) 0.1W 5wt% supermicroparticle of anhydrous silica and (c) water or a water-org. liquid mixture. The silane coupling agent to be used may be any one which can enhance adhesion between fluoroplastics and the substrate. The silane coupling agent is used as the aq. soln. and when it is difficulty soluble or insoluble in water, it may be used with addition of water after it is dissolved in a high- boiling hydrocarbon solvent such as toluene or xylene and then given affinity with water by use of a solvent such as ethanol or butanol easily soluble in water.