Storage-stable blocked polyisocyanates prepared by reacting one or more polyisocyanates with one or more biuretizing agents and optionally, catalysts such that in the blocked end product there are 5-45 equivalent percent of biuret groups according to formula (I) based on the sum total of all free and blocked NCO groups; optionally modifying the resulting biuret polyisocyanates with the aid of isocyanate-reactive compounds and/or catalysts, with further reaction of free NCO groups; and subsequently blocking at least 95 mol percent of the remaining free NCO groups with a blocking agent according to the formula R1R2NH, where R1 and R2 independently of one another are aliphatic or cycloaliphatic C1-C12 alkyl radicals. The blocked polyisocyanates can be used to make polyurethane polymers and coating compositions, which can be used to coat substrates.