Method for the in-situ deposition of a coating onto internal surfaces and matrix substrates of a body which defines one or more cavities. The single cavity, or the respective cavities packed with suitable substrates are provided with a predetermined amount of a solid reacting component to produce a suitable coating. The cavity is furnished with a liquid component which contacts the solid component, thereby forming a desired liquid coating reaction mixture. The latter is brought into contact with the cavity walls as well as with materials contained within the cavity, to achieve the desired coating of walls and matrix material.