Thanks for your comment.
So, i cut and scultp only plastic rods.
I size GW pieces when it's necessary.
See that for details :
http://www.coolminiornot.com/294570?browseid=1304009
or
http://www.frenchw40k.com/modules/newbb/viewtopic.php?start=20&topic_id=12317&viewmode=flat&order=ASC&type=&mode=0