Shaper Sheet

Aluminum takes paint fairly well if it isnt too shiny.(same for any metal really)

Primer, then pretty much "whatever".

An " etching" primer is best and shine wont matter if you can use it. 

Possible bonus, black or greys in primer should be pretty good for just dry brushing highlights over it depending on desired rock colors

 I would use the opposite side to paint on, not the Aluminum side. See the link below

https://ogrforum.com/...scene-lots-of-photos

Thanks, Alex

About 20 years ago, I always used aluminum foil for my scenery. I would cut lengths of about 2 feet long and take them outside and put them on the grass in the sun to spray them. I had four colors in spray cans, primer Brown , a medium green , black and light gray. I always started with the brown first. You had to be careful not to use to much black. It looked really good to me after glueing some lichen on.

Yes, foil is great, and easily painted. 

When I was building my mountain, I experimented with the old trick of using crumpled aluminum as a mold to make plaster rocks. Then I realized that this is crazy and just used the aluminum.  Duh! 

Alex M posted:

 I would use the opposite side to paint on, not the Aluminum side. See the link below

https://ogrforum.com/...scene-lots-of-photos

Thanks, Alex

I figure he had a reason.

But I am also curious why.....?

Thanks for all the responses. I'll build mountain with shaper sheet, add tunnel portals on both sides. I won't bother painting foil side.