A lot of people try and use just resin and Bondo. They usually regret it. Resin soaked paper is stiff, but not strong. The paper fibers aren't strong enough to support a lot. Bondo hardens things up, but it is very brittle. It is also heavy. People tend to make the Bondo thicker to increase the crack resistance, but it adds a lot of weight, is expensive, and is still brittle. By using fiberglass on the inside, you add a lot of strength without a lot of weight. You don't have to wear armor long to appreciate weight savings. By using Bondo over the fiberglass-strengthened Pepakura, you use less, save on sanding and repairs, save weight, and have armor that can withstand getting jostled while getting hauled around. Non-fiberglassed Pepakura is very at-risk when it is not on your person. Helmets in particular get banged around and I see lots of threads with busted helmets. Also, the helmet is the best area to save weight and reduce fatigue. Check out Vshore100's CQB thread. On of the nicest Pep builds out there. He just had devastating damage done to non-fiberglassed components.
Bondo: The less you use, the more you will love it. The more you use, the more you will hate it. It is very hard and not designed to sand or shape easily. Most noobs try to Bondo the whole thing at once or use lots of Bondo. Work small areas, and don't try to fill everything in one pass. It is much easier and cost-efficient to build up thin layers, sanding and filing a little between layers than building up and having to bust your ***** for hours removing material to get the shape and detail you want. Yes, everything will eventually have some Bondo on it to get a consistent surface for painting. Sanding the resined Pepakura directly will always be 'fuzzy' so you'll want Bondo as the upper surface.
Hope that helps.
Redshirt
