Howdy Russell,
First let me ask you if the "X" on the upper side is only visible with backlighting, meaning does it just show up as a thin area in the hood? If so, modelling putty would be the way to go to fill in the "X" on the bottom side. However, when dealing with real metal, "X's" are often 'pressed" into metal to strengthen it, this would not be a factor in plastic I wouldnt believe.
It may be that a "primer coat" of paint would cure your trouble on the top side. I would suggest a gray primer for light colors and a red oxide for darker color paint jobs.
Now as you may know, I often use regular spray paint on my models and also I often use "off the shelf" primer spray paints as well. I would suggest a cheap can of either the gray or red oxide for your model to cover any light that may come thru.
Having said that, it occurs to me that your problem may be a raised "X" on the topside of the hood. If this is the case, I would fill the "X" on the underside with putty, allow to dry and begin sanding the top side of the hood (you would also need to go back and sand the underside as well unless you plan on creating some sort of "insulation" like what is used on real automobiles in this area.
And other than what I have mentioned, I cant think of anything else at the moment w/o information on whether the "X" is raised or only translucent on the top side. I hope this helps.
Have a good day,
Greg
