An aqueous solution of manganese phosphate and phosphoric acid, preferably with little or no content of any material conventionally recognized as an accelerator or of any divalent metal ions other than manganese and iron, forms a good quality paint undercoating phosphate conversion coating on metal substrates, particularly on steel and galvanized steel substrates, when contacted by spraying or immersion with the substrates at a temperature of 54.degree.-65.degree. C. for a time of 2-5 minutes.