Ok ok, it's all my fault, i now can export with no artefacts on n-gones, i was just doing things the wrong way. Sorry to have make you lose time on this, but at least it made me understand...

For modo, yes i can select the inverted polys in modo, but i have to select them "from behind", wich means turning around and inside the mesh again and again. But anyway this issue was solved by closing the mesh in MoI, so no more need to spent time on this either.

So, many thanks for your help, it all came from me and my poor technical skills, but you had me progressing. :)

Back to work now, buildings, buildings, houses, buildings...