[AlexisAnabasis]

Must be: resistant to high temperatures/ pressures, non-toxic to humans, environmentally safe when washed away, natural ability to solidify as a smooth texture for coating purposes.

Preferred (non-essential) properties: Can be washed away by water or washed away by any other environmentally safe chemical.

Please let me know if you have any ideas or suggestions.

                       

[Enthalpy]

Welcome, AlexisAnabasis!

Alumina powder can be bought anhydrous, for use much like clay, after you add water. It should adhere on ceramics, less so on metals and polymers. Washed away, no chance.

Just clay might fit your needs. Not excellent for coatings. Once fired, it's hard but it's water resistant. Or plaster.

Would you detail what you call high temperature and high pressure? And how resistant shall the coating be, if water must wash it away? What materials shall it coat?

Epoxies resist moderate heat and are not very toxic nor very bad for the environment.