This innovation is relevant to a surface preparation of pre-fabricated steel parts, able to guarantee an excellent contact between the surface to be coated, and the molten bath, based on a Zinc-alloy containing Aluminum between 0.01 and 0.1 wt %. In a greater detail this invention is related to a procedure for hot-dip coat steel parts with a Zn—Al-alloy, according to which the parts are properly pickled and then immersed into an aqueous flux solution containing ZnCl2, NH4Cl, Bi2O3, and KCl, at a pH between 0.1 and 1.5 and a temperature in the range 4 and 50° C., for an immersion time between 10 s and 10 minutes. Using the flux solution according to the present innovation, it is possible to coat discontinuously with a Zn—Al-alloy, parts fabricated either with plain Carbon steels either high-strength steels. Furthermore, steels known commercially as Sandelin or Iper-sandelin, may be also coated without surface defects and with a glossy appearance.