Sandpaper doesn't get into the nooks very well, I can never get it to look right.
I'll try to get some pics up later, I'm playing catch up from surgery recovery this week.
My method was sandblast, mask, 2 thin coats tamiya fine white primer, 2 thin coats tamiya grey, then copper.
I use the tamiya white first, that way there is flat white to diffuse & reflect light inside the dish. Then the tamiya grey is to help with light blocking & give the copper a darker primer to sit on. Don't try to light block completely with the primer, it will be too thick. I finish up with copper, & that will finish off the light blocking at the same time.
There are recommendations for using silver or metallic paints for light blocking. Stay away from this, the metallic particles never fully seal in the paint & anything put on top of the metallics will peel off. Hence the copper coming off when you removed the little "dart" masks. Same thing happens when metallic paints are masked over without being sealed first. When the masks are removed the top shiny layer gets comes off, leaving a dull finish. Always use primers or stable colors under metallic.
Quick lighblocking tips:
This is the method I use on all models for light blocking.
A couple things first, flat white is the best surface for diffusing & reflecting light. Aluminum tape & chrome paint won't diffuse or reflect enough light & will cause light blooms, hot spots & uneven light distribution in the windows. Also they add a lot of weight to the model.
Paint & primer only, I don't use aluminum tape since the adhesive will eventually fail.
For the inside of models:
2 light coats of primer(white for clear parts, Black for opaque)
1 coat black spray paint
2 light coats of flat white
If you have a cheap tools store near. A small sandblasting gun can be picked up for around $20. If you're not using a cabinet, blast outdoors, ONLY use baking soda as the blasting media NO SAND! Always wear a respirator!
EL sheet is something I had considered, but there are a few cons. Lifespan is around 10,000hrs, compared to leds with 50,000-100,000hrs. Also there's the question of where to put the ballast which gives off a lot of heat.