It is fibre glass. Was a very tricky job. Had to cut away the body panel to get to do the required welding around the seatbelt mounting point. Rebuild the whole of the underneath of the car whilst we were at it, rebonded the bodywork and then a full respray. Quite a job! Also refurbed the alloys, repaired some small accident damage, sorted out the sagging doors (common prob on Alpines) and replaced arial.
Front wheel arch is prone to corrosion.
From inside. Pretty rotten!
This was the next car up after mine with a similar problem underneath!