A method of lining tubing within a wellbore comprises applying a polymerisable fluid composition containing a photoinitiator to the interior surface of that tubing and initiating polymerisation of the composition by exposing it to actinic radiation, suitably light or ultraviolet with wavelength 250 to 800 nm. The composition is preferably stable against heat but polymerises quickly when exposed to the actinic radiation. The composition may be spread onto the tubing and exposed to actinic radiation as soon as it has been spread into a layer, suitably with a tool which skims the tubing surface with applicator pads for dispensing and spreading the composition, immediately followed by exposing the spread composition to actinic radiation. A second option is that the composition is provided as a sleeve which is expanded against the tubing, exposure to actinic radiation preferably then being achieved using light guides or light emitting diodes within the sleeve.