OK - here's my method, passed on by Dave Boden of the Nottingham Club and shown to him by the late Phil Plater:
Grind and sand off all the high spots from the GRP and fill any large gaps or depressions with P38. Sand smooth and remove all traces of dust. Cut card templates of the inside of the bulwarks, to fit neatly along the line where the bulwarks meet the deck. Transfer the shapes to 0.5mm plasticard and cut out, leaving a surplus strip above the top edge for trimming back. Apply a VERY THIN smear of Evostick Impact (the smelly brown solvent-based one) to the inside surface of the bulwarks and the plasticard lining. If you put too much on at once you'll distort the GRP or plasticard; I apply a small dollop about the size of a 5p piece and smear it out straight away with my finger. Keep applying these smears until you've covered the surface evenly, then allow the Evostick to dry (about ten minutes). Carefully place the lining over the GRP, ensuring it lines up properly, then press it into place, working from the middle to the ends. Once it's all in position, give the combined bulwark and lining a good squeeze all over between finger and thumb to make sure there are no bubbles under the plastic. Finally trim the plastic back to the top of the GRP bulwark edges.
This method is used on all of my own Model Slipway models. It also has the advantage of allowing you to fit the bulwark supports and capping using liquid polystyrene glue, which I find much more forgiving than superglue.
Works for me every time, but suit yourself.