This invention relates to the simultaneous treatment of the internal and external surfaces of turbine blades or vanes. In particular it provides a process for coating an external and an internal surface of a turbine blade or vane with aluminium and chromium, respectively, at substantially the same time. The process comprises the following steps (i) and (ii) in either order: (i) applying to the external surface an aluminising compound comprising aluminium, a moderator, an energiser and a diluent; and (ii) applying to the internal surface a chromising compound comprising chromium, an energiser and a diluent. These steps are followed by (iii) heating the turbine blade or vane to form an aluminium layer on the external surface and a chromium layer on the internal surface. The invention also provides a suitable aluminising compound and a chromising compound per se.