You're exactly right about the billboards. They were an experiment to judge the correct printing size because they were cheap to make. Was it Delta Dog that had the tutorial with the sandpaper technique? Anyway, when I find that, I'm going to print out another batch, weather them up and then mount them on weathered bass wood. The bass wood will show through the holes in the sign and hopefully look fantastic.
I do highly recommend comic book backers for general papercraft projects because they're acid-free. The boards you see in the uncut sheets are literally 20 years old, no yellowing, no warping.
As for the sand, it's actually RR ballast. I don't plan on painting it, but I do plan on drizzling in more ballast in two other shades and adding a lot of 'post-apocalypse dirt' MIG weathering powder to even it out and make it look more natural. I want it to look buried in rubble, but it needs to look like it's been buried longer.
Also thinking of adding rust streaks on the plane, but I'm afraid of ruining it. I'm pretty pleased with the effect, but I'm one of those people that never finishes a miniature due to perfectionism...
Thanks for the input! I appreciate it. My gaming group is happy, but it seems a bit sycophantic as it's one of the best pieces we collectively own, and I'm shooting for a higher quality table overall.