What I do is that I use Tig´s Section Cut Face on my model then export 2 scenes from Layout: 1 - Sectioned model view; 2 - Section cut face scene with same camera as 1 but model is hidden. In layout I overlapp the two scenes where 2 is on top of 1. Then I can have lineweights for 1, different than 2 and still update the 2D drawing on the fly as I work on the model. I also add details to the section cut face, for construction detailing. When finished, if needed, I export to dwg from Layout.

@unknownuser said: It really is a shame about the rocks - as I'm planning to use this in lines I was hoping to be able to export them so a texture won't work - but I do see your point so I'll [l]eave these out. You could leave them out until the last moment before exporting the lines. And there's no point in using colors or textures in your model. Your rocks could be 2D, too. Just a sort of pattern of lines drawn on a face. @unknownuser said: Although even after deleting them my mode is still really slow ? There's still all those other things like the anchor bolts adding detail that has to get examined by the graphics card even though they don't show. If you have to have them, you might use simple proxies to stand in for them while modeling and swap them before the export. If you are going to show the whole model even on an A1 sheet, I doubt you'll be able to see the threads on those anchor bolts. You might try a simple test. In a new SKP file draw a rectangle the size of your city and add one of those anchor bolts. Make the export and print it on a sheet of A1 paper. Check out how much detail can be seen. Then revise the bolt to only have that much detail. By the way, do you know you could use LayOut and save the DWG export step? It can be set up to create A1 sheets. Export to PDF and print as needed.

Good luck.