I've been playing around with wonderflex for a year now and thought I  would give a little tutorial/WIP of my new Boodikka costume.
But  first an intro:
Wonderflex is a plastic that has a cheese cloth  like fabric embedded in the materiel.  It comes in sheets 43x55.  I get  mine from The Engineer Guy 
http://www.theengineerguy.com  for $39.99+shipping.  When you heat WF with a heat gun it emits a sticky  like substance and will stick to its self.  (it will also stick to nail  polish and some clothing but not ALL) This is really a great quality  since there is no other glues, ect that are needed.  One sheet of WF is  flexible, two sheets is pretty rigid and three sheets will withstand  quite a bit of pressure.  For example my Magdalena chest piece is one  sheet of WF with re-enforcement around the neck and arm holes.  The  sides are flexible and will bend around my ribcage.  The red in the  photo is leather.

WF will bend around curves and you can gently pull the heated substance  around shapes- like bewbs.  Or a halo.  This is wf over a Styrofoam  floral wreath.  This is the first heating shaping.  If you keep heating  WF it will smoosh into its self.

This is the 2nd heating where I smoothed it out more.  I use both wood  and metal clay shaping tools to smoosh and shape the wf.

After several coats of gesso and some paint you have a Dawn halo:

I first started learning about WF from Amethyst Angel's tutorial:  
http://www.amethyst-angel...wonderflex_tutorial.html  However like all tutorials your results may vary.  And mine did.   Friendly Plastic pellets are NOT the way to get a smooth finish....or at  least I couldn't.  Maybe a professional has a better way to use it, but  for now I classify Friendly Plastic as UNfriendly.
AA used a  paper mache balloon to get her shape.  I have made shapes two ways.  The  first way is to cut out a flat shape and then make notches or wedges  like sewing a dart in a woman's top.  The other way to get a shape is to  lay the WF over the shape and heat it up.  I use a combination of  both.  I make a pattern from paper on 1 inch square 3M flip chart  paper.  It sticks to the wall like a huge post it so I can hang it  anywhere I have room.  From there I cut out the pattern and try to shape  it.  If that works I then cut out the shape from Foamies.  Between tape  and heating the foamies I can get a better idea of how my wf piece is  going to shape up.  I get many of my armor patterns from SCA reenactment  armor- 
http://www.armourarchive.org/patterns/After  you have shaped your item you need to get rid of the grids.  
The new  and improved wf has one 'smooth' side, however the surface has a reptile  like texture to it.  Not so smooth in my mind but better than the  grids.There are several methods out there that people use on to  smooth out wf.
Friendly (NOT) Plastic - When you heat the pelletts  you end up heating the wf.  Only works if you are using 3 layers due to  heat.  One layer for my Magdalena shin armor did NOT work it was a god  awful mess.  It also really mucked up my chest armor.  It's very very very hard to get smooth.
Gesso- the kids on cosplay.com use this.  I think  it's because they are emo artist types and this is the only substance  anyone is familiar with, ergo it becomes cannon.  It's hard, very hard  but a PITA to sand, it streaks easy and it's expensive for how many  layers 5+ you have to use.
Wall Spackle - been there, done that,  made a mess.  BUT it was quick.  I put a layer of this over 2 layers of  Gesso.  But it's on a tiny piece of my armor on my upper arm that just  sorta hangs there.  I wouldn't use this on something that will get  knocked about/rubbed against.
Bondo or a Bondo product- I picked  this up at Auto Zone and will try it on an upcoming project.  BUT I  don't want to fuss with mixing.
Auto Filler Primer- I picked  this up at Auto Zone on a whim.  It is made to fill cracks on bumpers.   Plastic bumpers.  Fill cracks......DUH!  I haven't really looked back  since.  It takes two coats, dry, one more coat, dry, sand & DONE.
I  use nail sponges to sand- they are small and can get into cracks.  The  grit is 270.  You can get a huge pack of them at Sally's Beauty Supply  for around $8 bucks.

Here are several test strips to show the different smoothing methods.   The left is Auto Primer and it's as smooth as a baby's bum.
The  middle is gesso and the right is wall patch.  It is one coat of gesso  and one of wall patch.  After sanding you will have to apply a 2nd coat  of either.  To be honest I'm a busy lady and I like the no sanding  between coats thus the auto primer.
