trouble writing 3d extruded building to DXF

Hi

I want to write a 3D layer to DXF. The layer is extruded buildings from a 2D building layer, using an Extruder. When I write the output to DXF, it appears as a 3D object, but with triangles - screenshot attached.

Looking at the fme attributes, fme_type is fme_surface rather than solid, and fme_type is fme_aggregate. My understanding from reading some other posts is that I need the features to be solids not surfaces. Using the Geometry Coercer has no impact, it does not change it to a solid. And according to the help for the GeoemtryCoercer this is likely correct ("If you try to set the feature's geometry to fme_brep_solid and the geometry of the feature is not a composite surface or multi-surface or extrusion or box or csg solid, then the feature's geometry will be unchanged.").

Any ideas on how I set the feature to a solid? or what I need to set any autocad settings to - another post mentioned setting autocad_solid3d, but didn't give a value to set it to.

Thanks for any help

Mary