Thank you for your answer.
However as I mentioned earlier the issue with the walls isn't exclusive to bulldozer but remains when importing the object into arma. In one of the linked pages it says "If the geometry LOD is larger then the size limit, it gets glitchy (collision does not work for example and the object may disappear at certain view angles). The exact value of the limit is not yet known, but it is somewhere around 50-60 meter from the center of origin" however my model is at max 80 m long. i tried centering it on the grid in object builder and using wasd to move around in bulldozer but the problem seems to remain. I might try to import the correctly centered version into arma though at a later point. (Also I guess this refers to the object as a whole and not just parts of it.)
The LOD guide is very helpful to me, however I don't quite understand how I can rename the mesh. I got the Alt+L window open but don't know how to edit the mesh name. (it is probably so easy it isn't mentioned anywhere)