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.

Keep in mind that when skimming with body filler, you aren't trying to build thickness yet. All you want to do is fill the porosity of the open cells of foam, as well as any gaps and cracks that may be present in the foam. Pretty much as you did in the photo of your 'original' skim of body filler. So you're on the right track.