I worked For Rover cars Landrover and then BMW for 13 years , We did some Panel work for Nissan when they stopped building the micra and Almera we did all the spares that they are required too build. On the Doors we had too build we would take the Door skin put on pads very like the Roofing material and roller them on with as someone said here a wallpaper seaming roller then Apply a bead of sealing mastic around the edge of the door skin and anti rattle sealant on the tabs of the side impact bars holding the Doorskin tight. the whole thing was then Hydralic Pressed folding over the seam all the way round the door traping the inner too the outer. On my own Vehicles in the past I have used Hammerite underbody seal very effectivly but I used this too mainly seal inside panel gaps and seams as it remains flexible and wont peal and crack
