Another (reeeeal tedious) approach is to mask off the areas to the right and
left of one rib 'cell' - the space between two ribs - and gnetly sand/steel
wool the area. As you're sanding, the mask (I use black electricians' tape)
will 'protect' the area near the rib, so you end up with a shallow concave
cross section. This works very well on vacuform kits where the rib detail is
a simple raised line. However, I will admit it's time-consuming.