Apparatus for manufacturing a spirally wound conduit , from a metallic strip comprising an at least partially decorated surface finishing layer , the apparatus comprising: - shaping rollers configured to shape the longitudinal edges of the metal strip ; - driving rollers configured to feed the strip into a forming head having a cylindrical development for bringing the shaped longitudinal edges of the strip to engage with each other; - seaming rollers configured to seam the engaged edges each together and form a spiral lock-seam ; wherein said seaming rollers comprise a first seaming roller which comes into contact with the surface finishing layer comprising three portions: - a first cylindrical portion having a first diameter (d1); - a second cylindrical portion having a second diameter (d2); - a third cylindrical portion having a third diameter (d3); wherein said first diameter (d1) is greater than said second diameter (d2), said third diameter (d3) is greater than said second diameter (d2), and said second cylindrical portion is arranged between said first and third cylindrical portions ; and wherein said second cylindrical portion is joined to said first and third cylindrical portions by means of tapered sides (11', 11"); characterised in that : - said first diameter (d1) is greater than said third diameter (d3) and said third cylindrical portion is arranged towards the side of the apparatus from which the seamed conduit comes out; and a first driving roller that comes into contact with the surface finishing layer comprises a surface coating made of a polymeric material, preferably elastomeric, even more preferably polyurethane. - shaping rollers configured to shape the longitudinal edges of the metal strip ; - driving rollers configured to feed the strip into a forming head having a cylindrical development for bringing the shaped longitudinal edges of the strip to engage with each other; - seaming rollers configured to seam the engaged edges each together and form a spiral lock-seam ; wherein said seaming rollers comprise a first seaming roller which comes into contact with the surface finishing layer comprising three portions: - a first cylindrical portion having a first diameter (d1); - a second cylindrical portion having a second diameter (d2); - a third cylindrical portion having a third diameter (d3); - a first cylindrical portion having a first diameter (d1); - a second cylindrical portion having a second diameter (d2); - a third cylindrical portion having a third diameter (d3); wherein said first diameter (d1) is greater than said second diameter (d2), said third diameter (d3) is greater than said second diameter (d2), and said second cylindrical portion is arranged between said first and third cylindrical portions ; and wherein said second cylindrical portion is joined to said first and third cylindrical portions by means of tapered sides (11', 11"); characterised in that : - said first diameter (d1) is greater than said third diameter (d3) and said third cylindrical portion is arranged towards the side of the apparatus from which the seamed conduit comes out; and - said first diameter (d1) is greater than said third diameter (d3) and said third cylindrical portion is arranged towards the side of the apparatus from which the seamed conduit comes out; and a first driving roller that comes into contact with the surface finishing layer comprises a surface coating made of a polymeric material, preferably elastomeric, even more preferably polyurethane. <EOS>