Apparatus for applying a sealant to a surface of an object, said apparatus comprising: - at least one annular channel for supplying plasticised material, said annular channel having at least one longitudinal axis; - one annular outlet of said annular channel shaped in such a manner that the outlet direction of the plasticised material has at least one radial component that is normal to said longitudinal axis; - at least one core that bounds internally said annular channel; - at least one tubular wall that bounds externally said annular channel, said tubular wall and said core being movable in relation to one another with the possibility of adopting at least one open position and a closing position of said annular outlet; in said closing position, said tubular wall having at least one annular plug surface that interacts with one annular plug seat arranged on said core to close said annular outlet forming one annular dose with the plasticised material that has exited said annular outlet; in said open position, said annular outlet having a side bounded by said plug surface and an opposite side bounded by said plug seat; - driving means for performing opening and closing cycles of said annular outlet to form at every cycle at least one annular dose; - means for depositing the annular dose on the surface of the object, said depositing means comprising supporting means for supporting the object having the surface with the possibility of adopting a receiving position in which it is near said annular outlet so that the annular dose that has just been formed is in contact with the surface; - compression-forming means (21; 22) for moulding the annular dose on the surface of the object; wherein: - said tubular wall has at least one annular internal distal surface that is contiguous with said plug surface inside said annular channel, in said closing position said internal distal surface being spaced away from said core, at least one part of said internal distal surface facing axially, i.e. in a direction that is parallel to said longitudinal axis, to an internal surface of said core that is contiguous with said plug seat inside said annular channel, defining in said closing position one annular gap that is interposed between said internal distal surface and said internal surface inside said annular channel; characterised in that: - IN SAID CLOSING POSITION, said plug surface is in contact with said plug seat with a radial width, i.e. a width component measured in a direction that is normal to said longitudinal axis, that is non-null and less than 0.2 mm. - at least one annular channel for supplying plasticised material, said annular channel having at least one longitudinal axis; - one annular outlet of said annular channel shaped in such a manner that the outlet direction of the plasticised material has at least one radial component that is normal to said longitudinal axis; - at least one core that bounds internally said annular channel; - at least one tubular wall that bounds externally said annular channel, said tubular wall and said core being movable in relation to one another with the possibility of adopting at least one open position and a closing position of said annular outlet; in said closing position, said tubular wall having at least one annular plug surface that interacts with one annular plug seat arranged on said core to close said annular outlet forming one annular dose with the plasticised material that has exited said annular outlet; in said open position, said annular outlet having a side bounded by said plug surface and an opposite side bounded by said plug seat; - driving means for performing opening and closing cycles of said annular outlet to form at every cycle at least one annular dose; - means for depositing the annular dose on the surface of the object, said depositing means comprising supporting means for supporting the object having the surface with the possibility of adopting a receiving position in which it is near said annular outlet so that the annular dose that has just been formed is in contact with the surface; - compression-forming means (21; 22) for moulding the annular dose on the surface of the object; - said tubular wall has at least one annular internal distal surface that is contiguous with said plug surface inside said annular channel, in said closing position said internal distal surface being spaced away from said core, at least one part of said internal distal surface facing axially, i.e. in a direction that is parallel to said longitudinal axis, to an internal surface of said core that is contiguous with said plug seat inside said annular channel, defining in said closing position one annular gap that is interposed between said internal distal surface and said internal surface inside said annular channel; - IN SAID CLOSING POSITION, said plug surface is in contact with said plug seat with a radial width, i.e. a width component measured in a direction that is normal to said longitudinal axis, that is non-null and less than 0.2 mm.