To get the "plasticy" look, you need to keep working your surfaces, work and work and work. Get your bondo on smoothly and take it back with a heavy grit, then bondo again and repeat, keep working untill you have a smooth surface and work your way up to finer grits. What might be a really good idea for you to try, could be to buy one of the hasbro kids helmets and practice using bondo and sanding on it. Pepakura is not a good way to learn prop making. There is a misconception that it's a good starting point for people when in reality it's actually quite an advanced model making tool. You need quite a wide knowledge base to get a professional result from it. You could liken it to airplanes, a lump of clay is like a Cessna, a great training tool and perfect starting point and, in the hands of a pro it can do wonderful things. Pepaakura is like the space shuttle, it looks like a plane but, really it's nothing like one, it's an experimental aircraft, if something goes wrong, no one walks away, you're left with a crater. My point is, with pep you're working a ridged surface, if you structure is out by a tiny bit, the whole thing is ruined, it will never look good. You can sand all you like but you'll just be polishing a turd. Clay is far more forgiving. You can keep going back to base structure and altering it without too much drama.
I hope that helps