Wonderflex:
CosplaySupplies.com -- Wonder Flex
For those that dont know what wonderflex is, it is along the lines of plastic.
Wonderflex is a felt like material that has all the characteristics of felt fabric, but when heated past a certain point, special chemicals release, and upon cooling, harden into stiff plastic. It also bonds to itself during the heating stage which is good for smooth seamless armor.
A massive role of wonderflex (Wondeflex Jumbo Sheet 40" x 55" / 101 cm by 139 cm) costs $55. I love this site, its where i've gotten stuff for all anime related cosplays, and its what im using for my armor.
Similar product:
CosplaySupplies.com -- Friendly Plastic
"Drop these white pellets into warm water for just a few seconds to melt. Friendly Plastic® pellets are completely reusable. If you don't like how the shape turned out, simply re-soften and start over. Useful for coating and smoothing Wonderflex."
It works best for making wonderflex seamless, but if you really want, you could make a pepakura mold of the helm or what not, and pour the melted pellets into it, to use the (card stock) paper as a negative mold. Use sand paper or a hot rag to soften the plastic and smooth it.
Plasti Dip
A spray on synthetic rubber material that can withstand up to negative 30 degrees F, and over 200 degrees F before cracking or warping. It resists tearing and comes in a spray and paint.
I suggest both spray and paint. Start off with 3 layers of the paint to make smooth surfaces, then do sprays more perfectly even coats. Spraying at the beginning may allow for dimples or seams to show through.
Can be found at menards or ace hardware.
Hope you find it helpful, maybe add the links to first post.