The invention relates to 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.