Lots of thin coats of resin are the best bet. Don't glob it on. Just do 3-5 thin coats, then use Bondo Boddy filler to fill all of the exposed areas of the edge of the cardboard. It takes time, but works. (Used this on my cardboard BR, and so far, its awesome!) Just make sure you sand the resin befor bondoing, and then add 1 more coat of resin where needed after bondo. From there, sand one more time, sand again to be extra smooth, and prime/paint it.
Ive been doing this method for 2 years with no problems at all. The gun always turns out great. Trust me, it will work if you be patient, and always pay close attention to your work. Add small details that you think you could add with bondo and cardboard. Scratch build. Its always the best way to go.
CHEERS!