How do you give cardboard a good clean smooth finish?

I recently finished a part made out of cardboard, and I wanted to know how to give it a finish from cardboard. I can only do it from the outside. And i only have body filler, resin, and fiberglass. Just saying because they might be useful somehow.

Anyway, are there any methods out there to give cardboard this clean finish? So it is paintable.

Thanks in advance.

I don't know whether you have used corrigated or regular cardstock. But this has been my method with cardstock.

I usually paint on a few coats of a mixture of white glue and water with sandings between coats. The water helps absorb the glue into the fibers, but you don't want to over saturate your project with it.

After a few dried coats and the final sanding, I've spraypainted a few coats of polyurethane to give it a tougher skin, after curing spraypaint on the color of choice, dry, then followed with either the urethane or some other clear coat.

Hope this helps out.