Being a good boy, and because I knew things would go wrong if I didn't, the railcar body was left overnight to harden properly before I attempted to sort out the joints and other imperfections.
Fortunately the previous owner had already removed all the raised lining lines moulded into the surface by Airfix. The job wasn't shabby either - when I try this I usually excavate chunks of plastic that should be retained. Obviously care and a good sharp blade had been used to good effect.
All I needed to do was smooth the corner joins and polish the plastic up a big. When doing this, emery paper on it's own isn't a good thing as fingers have a tendency to make hollows in the surface due to inconsistent pressure being applied. A better plan is to attach the abrasive to a board.
Better still, these smoothing sticks came from a tool shop in Birmingham's Jewellery Quarter although I think you can get them from Squires now. Each is fitted with a different grade of emery - the black ones are various flavours of medium and the pink ones very fine. Not the cheapest tools as I recall but very good for this sort of task and I now have very smooth joints.