A process for bonding a pair of sheet moulding compound substrates, comprising a) mixing together a resin component A and a curative B of a two-component polyurethane system at a mixing ratio such that the mixture contains an excess of isocyanate groups, to form a heat-curable adhesive; b) applying the heat curable adhesive to a bondline intermediate to the sheet moulding compound substrates; and c) heating the heat-curable adhesive to a temperature of at least that at which the blocked aza catalyst becomes activated to cure the heat-curable adhesive and form an adhesive bond between the sheet moulding compound substrates at the site of the bondline, wherein the resin component A and the curative B are separately packaged, each of resin component A and curative B are liquids having a Brookfield viscosity of from 5,000 to 50,000 cps at 25°C, measured using a 7 spindle at 10 rpm at 25°C, wherein: the resin component A has an isocyanate equivalent weight of from 230 to 350 and comprises (A-1) a mixture of isocyanate compounds, which mixture is liquid at 25°C, has an average isocyanate content of from 15 to 23% by weight and includes a monomeric polyisocyanate compound containing from 2 to 4 isocyanate groups per molecule, and which is devoid of urethane or urea linkages and a prepolymer which is a reaction product of at least one polyisocyanate and at least one isocyanate-reactive compound that includes a difunctional polyether polyol having a molecular weight of from 1000 to 3000 and (A-2) at least one tin catalyst, and the curative B has an equivalent weight per isocyanate-reactive group of from 250 to 450 and comprises (B-1) at least one polyether polyol which contains from 2 to 4 hydroxyl groups per molecule and has a number average molecular weight of from 3000 to 20,000; (B-2) at least one polyol having from 4 to 8 hydroxyl groups per molecule and a molecular weight of from 200 to 1000 and (B-3) at least one blocked aza catalyst which is heat activatable at a temperature of at least 65°C. a) mixing together a resin component A and a curative B of a two-component polyurethane system at a mixing ratio such that the mixture contains an excess of isocyanate groups, to form a heat-curable adhesive; b) applying the heat curable adhesive to a bondline intermediate to the sheet moulding compound substrates; and c) heating the heat-curable adhesive to a temperature of at least that at which the blocked aza catalyst becomes activated to cure the heat-curable adhesive and form an adhesive bond between the sheet moulding compound substrates at the site of the bondline, wherein the resin component A and the curative B are separately packaged, each of resin component A and curative B are liquids having a Brookfield viscosity of from 5,000 to 50,000 cps at 25°C, measured using a 7 spindle at 10 rpm at 25°C, wherein: the resin component A has an isocyanate equivalent weight of from 230 to 350 and comprises (A-1) a mixture of isocyanate compounds, which mixture is liquid at 25°C, has an average isocyanate content of from 15 to 23% by weight and includes a monomeric polyisocyanate compound containing from 2 to 4 isocyanate groups per molecule, and which is devoid of urethane or urea linkages and a prepolymer which is a reaction product of at least one polyisocyanate and at least one isocyanate-reactive compound that includes a difunctional polyether polyol having a molecular weight of from 1000 to 3000 and (A-2) at least one tin catalyst, and the curative B has an equivalent weight per isocyanate-reactive group of from 250 to 450 and comprises (B-1) at least one polyether polyol which contains from 2 to 4 hydroxyl groups per molecule and has a number average molecular weight of from 3000 to 20,000; (B-2) at least one polyol having from 4 to 8 hydroxyl groups per molecule and a molecular weight of from 200 to 1000 and (B-3) at least one blocked aza catalyst which is heat activatable at a temperature of at least 65°C. the resin component A has an isocyanate equivalent weight of from 230 to 350 and comprises (A-1) a mixture of isocyanate compounds, which mixture is liquid at 25°C, has an average isocyanate content of from 15 to 23% by weight and includes a monomeric polyisocyanate compound containing from 2 to 4 isocyanate groups per molecule, and which is devoid of urethane or urea linkages and a prepolymer which is a reaction product of at least one polyisocyanate and at least one isocyanate-reactive compound that includes a difunctional polyether polyol having a molecular weight of from 1000 to 3000 and (A-2) at least one tin catalyst, and the curative B has an equivalent weight per isocyanate-reactive group of from 250 to 450 and comprises (B-1) at least one polyether polyol which contains from 2 to 4 hydroxyl groups per molecule and has a number average molecular weight of from 3000 to 20,000; (B-2) at least one polyol having from 4 to 8 hydroxyl groups per molecule and a molecular weight of from 200 to 1000 and (B-3) at least one blocked aza catalyst which is heat activatable at a temperature of at least 65°C.