Most people use Bondo brand fiberglass resin. Like
marinesniper said, light layers at first to help prevent warping. It is also recommended to make your paper models out of 110 weight cardstock to prevent warping. After resin, you can use fiberglass mat/cloth coated in the same resin to strengthen. Another option is to mix resin and Bondo auto body filler to make what is commonly known as "rondo". It is easier to use and less messy but is less strong than fiberglass. Then it's Bondo auto body filler outside for shaping and sanding to make everything smooth and uniform.
Personally, I do 1 thin layer of resin inside and out then 1 or 2 thicker layers of resin. I then fiberglass the inside AND do a layer of rondo over the fiberglass. That way it's very strong and the pokey fiberglass is covered with a smooth coating. People generally consider rondo and fiberglass overkill tho.
There are also specific tutorials in the tutorials section of the forum that have more specific details. Use the index to find exactly what you need, but you can start here:
The following thread contains a series of videos that I have created and compiled into one place. They provide a step by step look at my on-going build of Spartan Mk VI Armor. The videos are created with a "Tutorial" feel and should be regarded as "Examples" on how to proceed... As new videos...
www.405th.com