I've had a look, but couldn't find anything about this. Has anyone removed the gutter seams on a mk2 saloon? I've smoothed other shells (mainly minis), reenforcing and welding the joins before cutting the rails off. Has anyone tried it or is there a reason why it's an absolute no no?

I think a few got done a few years ago when smoothing was the in thing. I've never done it myself, but it should be possible. I suppose the benefit on a mini is you don't have the angle at the edge of the roof skin, and the seam is thicker in the first place, whereas on a polo, there's not a lot between the tops of the windows and the edge of the roof.