I don't want to come across as an expert on this (but that never stopped me before
), but...
a body long-board has a turnbuckle on the back so you can warp it from [slightly] concave to [slightly] convex.
It might be worth it to make custom sanding blocks for the edges, to keep them uniform.
With paint you can feel irregularities with your fingers that you can't see with you eye. Probably not on soft foam, but you don't want to hold the sandpaper without a block.
If you haven't used guide coats before, they don't have to be more than a mist of paint. If you knock down the highs without hollowing out the lows less filler is needed.
Is this form male or female? Or was it that you haven't decided?