A continuous process for coating ferrous or zinciferous metal substrates using an acidic coating solution of zinc phosphate in the presence of hydrogen peroxide wherein (a) the molar ratio ortho phosphate/zinc in solution is maintained in the range 0.5-3.7, and (b) the coating solution is replenished with hydrogen peroxide, replenishment feed (i) (comprising zinc, ortho phosphate and anion N.sup.n-), and replenishment feed (ii) (comprising alkali), the ratio of the quantities of (i) and (ii) being defined. The acid H.sub.n N has a pK.sub.a value not greater than 3.