The invention relates to a method of producing a flexible pipe comprising a metal carcass and an internal sealing sheath extruded onto the carcass, said 5 method comprising providing a metal carcass; heating an application section of said carcass, preferably using induction heating, to an application section temperature of at least 150° C.; extruding a non-cross-linked polyethylene material comprising a peroxide having an activation temperature above 150° C. onto said application section of said carcass; cross-linking the extruded 10 polyethylene in a cross-linking zone by raising its temperature to at least the activation temperature of said peroxide by exposing the extruded polymer material to electromagnetic waves, with a wavelength of between 0.5 μm to 0.5 m, preferably infrared radiation; and cooling said cross-linked polyethylene material to obtain the internal sealing sheath. 15 In a preferred embodiment the method comprises heating the application section to an application section temperature of between 30 and 5° C. below the activation temperature of the peroxide. 20 Preferred peroxides include butylcumyl peroxide, dicumyl peroxide, 2,5-Dimethyl-2,5-di(tert-butylperoxy)hexyne-3,3,3,5,7,7-Pentamethyl-1,2,4-trioxepane, hydroperoxide, 2,5-dimethyl hexane 2,5-di-t-butyl peroxide, bis(t-butylperoxy isopropyl)benzene, t-butyl cumyl peroxide, di-t-butyl peroxide, 2,5-dimethyl hexine-3 2,5-di-t-butyl peroxide and butylhydroperoxide. 25 A foil may be applied onto the metal carcass prior to the step of heating the metal carcass, the polyethylene material being extruded onto said foil. 30