Quoted from timab2000:
I was just looking at the back glass for my lost world some of the artwork and Things That You Don't See right off the bat unless you really start looking at it it's pretty amazing
Bally sold tons of the machine back in 78 because of that bg artwork - no one had seen anything like it previously. I think it was in an interview with Paul Faris where he mentions this. The 4 color process allowing them to reproduce paintings on backglasses really made their games stand out from others with the plain line art (although some very good artists like Dave Christensen certainly was a master of the line art).
Is the primer you are using supposed to seal the wood as well? I always thought sealer first followed by primer if they aren't one and the same.