I want to build a coro wing with a specific airfoil. NACA 4412 to be precise. How can i make sure that the wing has the same airfoil throughout?

Difficult. Make ribs, you can get quite close, but definitely not 100%

Also, what will be the weight compared to standard balsa construction with ribs?

Balsa will be much lighter!

You cannot get the exact NACA 4412 profile if you use coro sheet. Since coro sheet has flute which will compress and will not give you the exact profile you need.

But you can try. You need to make NACA 4412 ribs and fix it to the spar as close as possible and cover the coro sheet such that the flute is parallel to spar. But covering coro sheet like this has less strength. You need to have strong spar and ribs to make your wing rigid.

Balsa Ribs:   (with foam filling in front part)  http://www.rcindia.org/electric-planes/the-hots-in-corro-a-bridge-too-far/

Instead you can cover it with covering film instead of coro sheet. This will give you near profile and much lesser weight

With full balsa ribs

http://www.rcindia.org/electric-planes/16-piper-cub-(svenson)-in-corro/25/

Thanks everyone!