If it's of pine, leave the inside raw, and paint, stain, varnish the outside as you wish!
Just make sure it's dry for a week so the chins don't smell the fumes, and do it well away from them for the same reason!
The only thing safe for wire cages is powder coating - if you google it, and ask them to do it when they're doing a large run, [if your not picky on the color] it shouldn't be too expensive!
Please post pictures when done!!