Never import a DWG file.
And never, ever explode an Imported DWG file.
Nothing good will come of it. Trust me. Instead, do this (in order of preference):
- Use Revit natively, without adding messy DWG or CAD import data.
- Link DWG files, rather than import.
- Import DWG files into a Detail Component family, and then load that family in.
- Eat a very sour lemon.
- Import DWG file.
- Import DWG file and explode it.