depending on just how dusty-crusty they are,I have had good success with this method:
Using a fine media like glass or fine sand,stay back a good distance and lightly and quickly blast the surface.It is amazing how the dry dusty junk disappears,and leaves the grain intact.Then wash,clean etc thouroughly before respraying with SEM or other dyes.This also works well on kick panels and A pillar mouldings.Don't get to close or agressive when blasting !!Of course this method depends on just how bad the panels are,but I have done this many times with good results.