A method for reinforcing a part of the outer surface of a wind turbine blade, said method comprises the steps: i) providing a blade plug having an outer surface resembling the topography of the outer surface of at least a leading portion of at least part of the length of a wind turbine blade; ii) casting a mold of part of the blade plug obtained in step i) in such a way that the topography of an inner surface of said mold corresponds to the topography of part of an outer surface of said blade plug provided in step i); iii) from the mold obtained in step ii), preparing a protective shell by making a casting of the inner surface of said mold; said protective shell is comprising an inner surface and an outer surface, said protective shell is being made from one or more predetermined materials; iv) starting from the topography of the surface of the wind turbine blade; or starting from a blade plug as obtained in step i) preparing an enlarged plug; said enlarged plug thereby comprising an outer surface resembling the topography of the outer surface of at least a leading part of said wind turbine blade; said outer surface of said enlarged plug is having larger dimensions than said outer surface of said blade plug; v) from the enlarged plug obtained in step iv), casting a mounting shell having an inner surface and an outer surface, in such a way that the topography of at least part of an inner surface of said mounting shell corresponds to the topography of part of an outer surface of said enlarged plug; vi) applying an adhesive to at least part of the inner surface of said protective shell and/or to at least part of the outer surface of at least a leading portion of said outer surface of said wind turbine blade; vii) fitting the inner surface of said protective shell onto at least a leading portion of the outer surface of said wind turbine blade; viii) fitting the inner surface of said mounting shell onto said outer surface of said protective shell; ix) applying a force to said mounting shell, and thereby also to said outer surface of said protective shell; wherein said force comprises a force component in a cord direction from the leading surface to the trailing surface of said wind turbine blade; wherein said force additionally comprises a force component in a direction perpendicular to the cord direction and perpendicular to the lengthwise direction of said wind turbine blade; thereby pressing said mounting shell and said protective shell against the outer surface of the wind turbine blade. x) allowing said adhesive applied in step vi) to cure, and subsequently removing said mounting shell from said wind turbine blade and from said protective shell. i) providing a blade plug having an outer surface resembling the topography of the outer surface of at least a leading portion of at least part of the length of a wind turbine blade; ii) casting a mold of part of the blade plug obtained in step i) in such a way that the topography of an inner surface of said mold corresponds to the topography of part of an outer surface of said blade plug provided in step i); iii) from the mold obtained in step ii), preparing a protective shell by making a casting of the inner surface of said mold; said protective shell is comprising an inner surface and an outer surface, said protective shell is being made from one or more predetermined materials; iv) starting from the topography of the surface of the wind turbine blade; or starting from a blade plug as obtained in step i) preparing an enlarged plug; said enlarged plug thereby comprising an outer surface resembling the topography of the outer surface of at least a leading part of said wind turbine blade; said outer surface of said enlarged plug is having larger dimensions than said outer surface of said blade plug; v) from the enlarged plug obtained in step iv), casting a mounting shell having an inner surface and an outer surface, in such a way that the topography of at least part of an inner surface of said mounting shell corresponds to the topography of part of an outer surface of said enlarged plug; vi) applying an adhesive to at least part of the inner surface of said protective shell and/or to at least part of the outer surface of at least a leading portion of said outer surface of said wind turbine blade; vii) fitting the inner surface of said protective shell onto at least a leading portion of the outer surface of said wind turbine blade; viii) fitting the inner surface of said mounting shell onto said outer surface of said protective shell; ix) applying a force to said mounting shell, and thereby also to said outer surface of said protective shell; wherein said force comprises a force component in a cord direction from the leading surface to the trailing surface of said wind turbine blade; wherein said force additionally comprises a force component in a direction perpendicular to the cord direction and perpendicular to the lengthwise direction of said wind turbine blade; thereby pressing said mounting shell and said protective shell against the outer surface of the wind turbine blade. x) allowing said adhesive applied in step vi) to cure, and subsequently removing said mounting shell from said wind turbine blade and from said protective shell.