Is it safe to use a pressure washer on a marble wall? The wall is located outside a building?

The use of a pressure washer on marble should be avoided. Pressure washing forces water into the pores of the stone and can cause the development of efflorescence. If you must use a pressure washer make sure that the operator is skilled. Do not use a zero tip point and avoid pressures over 1500 psi.

CAUTION Certain stones are very brittle and pressure washing may weaken them causing rapid deterioration. Shell stone, coquina, travertine and breccia can be damaged by pressure washing very easily.