Method for forming multilayer coating film

1. A method for forming a multilayer coating film, comprising:

an intermediate coating step of applying an aqueous intermediate coating composition (A) to a surface of an article to be coated to form an uncured aqueous intermediate coating film;

a first aqueous base coating step of applying a first aqueous base coating composition (B) onto the uncured aqueous intermediate coating film to form an uncured first aqueous base coating film;

a second aqueous base coating step of applying a second aqueous base coating composition (C) containing a scaly luster pigment onto the uncured first aqueous base coating film to form an uncured second aqueous base coating film;

a clear coating step of applying a clear coating composition (D) onto the uncured second aqueous base coating film to form an uncured clear coating film; and

a curing step of heat curing at once the uncured aqueous intermediate coating film, the uncured first aqueous base coating film, the uncured second aqueous base coating film, and the uncured clear coating film each obtained in the preceding steps to form a multilayer coating film,

wherein a solid concentration NV mB (% by mass) attained at 1 minute after coating, of the uncured first aqueous base coating film formed in the first aqueous base coating step and a solid concentration NV sB (% by mass) attained at 1 minute after coating, of an uncured first aqueous base coating film formed by applying the first aqueous base coating composition (B) directly to a substrate having no water absorption property satisfy formula:

1.2 ≤NV mB /NV sB , and

the solid concentration NV mB (% by mass) and a solid concentration NV sC (% by mass) attained at 1 minute after coating, of an uncured second aqueous base coating film formed by applying the second aqueous base coating composition (C) directly to the substrate having no water absorption property satisfy formula:

NV mB (% by mass)− NV sC (% by mass)≥20 (% by mass).

2. The method for forming a multilayer coating film according to claim 1 , wherein the uncured first aqueous base coating film formed in the first aqueous base coating step has a viscosity η mB attained at 1 minute after coating of 270 Pa·s or more.

3. The method for forming a multilayer coating film according to claim 2 , wherein

the solid concentration NV sB (% by mass) attained at 1 minute after coating, of the uncured first aqueous base coating film formed by applying the first aqueous base coating composition (B) directly to the substrate having no water absorption property is 20% by mass or more and 40% by mass or less, and

a viscosity η sB attained at 1 minute after coating, of the uncured first aqueous base coating film formed by applying the first aqueous base coating composition (B) directly to the substrate having no water absorption property is in a range of 30 to 300 Pa·s.

4. The method for forming a multilayer coating film according to claim 2 , wherein the solid concentration NV sC (% by mass) attained at 1 minute after coating, of an uncured second aqueous base coating film formed by applying the second aqueous base coating composition (C) directly to the substrate having no water absorption property is 10% by mass or more and 25% by mass or less.

5. The method for forming a multilayer coating film according to claim 2 , wherein a solid concentration NV mA (% by mass) of the uncured aqueous intermediate coating film in the first aqueous base coating step is 80% by mass or more.

6. The method for forming a multilayer coating film according to claim 2 , wherein an amount of the scaly luster pigment contained in the second aqueous base coating composition (C) is in a range of 20 to 40 parts by mass per 100 parts by mass of the resin solid content of the second aqueous base coating composition (C).

7. The method for forming a multilayer coating film according to claim 1 , wherein

the solid concentration NV sB (% by mass) attained at 1 minute after coating, of the uncured first aqueous base coating film formed by applying the first aqueous base coating composition (B) directly to the substrate having no water absorption property is 20% by mass or more and 40% by mass or less, and

a viscosity η sB attained at 1 minute after coating, of the uncured first aqueous base coating film formed by applying the first aqueous base coating composition (B) directly to the substrate having no water absorption property is in a range of 30 to 300 Pa·s.

8. The method for forming a multilayer coating film according to claim 7 , wherein the solid concentration NV sC (% by mass) attained at 1 minute after coating, of an uncured second aqueous base coating film formed by applying the second aqueous base coating composition (C) directly to the substrate having no water absorption property is 10% by mass or more and 25% by mass or less.

9. The method for forming a multilayer coating film according to claim 7 , wherein a solid concentration NV mA (% by mass) of the uncured aqueous intermediate coating film in the first aqueous base coating step is 80% by mass or more.

10. The method for forming a multilayer coating film according to claim 7 , wherein an amount of the scaly luster pigment contained in the second aqueous base coating composition (C) is in a range of 20 to 40 parts by mass per 100 parts by mass of the resin solid content of the second aqueous base coating composition (C).

11. The method for forming a multilayer coating film according to claim 1 , wherein the solid concentration NV sC (% by mass) attained at 1 minute after coating, of an uncured second aqueous base coating film formed by applying the second aqueous base coating composition (C) directly to the substrate having no water absorption property is 10% by mass or more and 25% by mass or less.

12. The method for forming a multilayer coating film according to claim 11 , wherein a solid concentration NV mA (% by mass) of the uncured aqueous intermediate coating film in the first aqueous base coating step is 80% by mass or more.

13. The method for forming a multilayer coating film according to claim 11 , wherein an amount of the scaly luster pigment contained in the second aqueous base coating composition (C) is in a range of 20 to 40 parts by mass per 100 parts by mass of the resin solid content of the second aqueous base coating composition (C).

14. The method for forming a multilayer coating film according to claim 1 , wherein a solid concentration NV mA (% by mass) of the uncured aqueous intermediate coating film in the first aqueous base coating step is 80% by mass or more.

15. The method for forming a multilayer coating film according to claim 14 , wherein an amount of the scaly luster pigment contained in the second aqueous base coating composition (C) is in a range of 20 to 40 parts by mass per 100 parts by mass of the resin solid content of the second aqueous base coating composition (C).

16. The method for forming a multilayer coating film according to claim 1 , wherein an amount of the scaly luster pigment contained in the second aqueous base coating composition (C) is in a range of 20 to 40 parts by mass per 100 parts by mass of the resin solid content of the second aqueous base coating composition (C).