Water-dilutable coating agent for preparing the base layer of a multilayer coating

A process for preparing a multilayer coating, which comprises coating a substrate with a base layer of a waterdilutable coating agent which contains pigments and which contains as a binder an aqueous polyurethane dispersion which has been prepared by reacting

• (A) a linear polyether diol and/or polyester diol having terminal hydroxyl groups and a molecular weight of 400 to 3,000 with
• (B) a diisocyanate to form a first intermediate product in such a molar ratio that said intermediate product has terminal isocyanate groups;
• (C) and reacting said first intermediate product with a compound which has two groups which are reactive toward isocyanate groups and at least one group which is capable of forming an anion, said compound neutralized with a tertiary amine before the reaction and forming a second intermediate product, converting said second intermediate product, obtained from (A), (B) and (C), into a predominantly aqueous phase;
• and
• (D) reacting isocyanate groups which are still present with a diamine and/or polyamine having primary and/or secondary amine groups, applying a top coat of a transparent coating agent after an air drying period and then heating the coated substrate and conjointly hardening the coating layers.

A substrate coated with a multilayer coating which has been obtained by applying a water-dilutable coating agent which contained pigments and by applying a transparent coating agent as a top coat and subsequently heating the coated substrate, in which the coating agent for the base layer contained as binder an aqueous polyurethane dispersion having a polyurethane resin acid number of 5 to 70 and having been prepared by reacting

• (A) a linear polyether diol and/or polyester diol having terminal hydroxyl groups and a molecular weight of 400 to 3,000 with
• (B) a diisocyanate to form a first intermediate product in such a molar ratio that said first intermediate product has terminal isocyanate groups;
• (C) and reacting the intermediate product with a compound which has two groups which are reactive toward isocyanate groups and at least one group which is capable of forming an anion, said compound neutralized with a tertiary amine before the reaction and forming a second intermediate product, converting said second intermediate product, obtained from (A), (B) and (C), into a predominantly aqueous phase;
• and
• (D) reacting isocyanate groups which are still present with a diamine and/or polyamine having primary and/or secondary amino groups.