I'm in need of a little help guys  ........  I watched ( and learned a great deal ) from Paul's 'Airbox' video on YouTube,  I purchased and currently using the Low Density 48Kgs/m3 PU foam blocks, to get my 'form'.  I then put a skim of body filler to harden the 'form' that will eventually be my plug for casting a resin mould.  After the initial body filler 'skim' had hardened,  I applied a few more layers of body filler to build-up the form to the dimensions I wanted.  So basically,  my plug now has about 3mm of body filler on it that needs to be sanded down smooth.