I'm assuming you used a closed cell foam (eg. Sondor SPX60 ) to make your plug/mould ?
That foam is polyethylene based. Resin eats it pretty hectically.
It doesn't take any kind of filler material or wax well.
We've experimented with it in our pattern shop for tooling purposes.
The best way forward on that will be to clad the foam in a layer of tin foil (shiny side up).
Spread a thin layer of contact adhesive type glue over the foam and apply the tin foil while it is wet. Spread out any folds or blemishes in the foil like you were "ironing" it, so that you have an almost mirror type finish.
Once everything has dried, brush a thin layer of resin onto the tin foil until it is tacky.
Lay a layer of epoxy cloth on the resin and apply resin over the cloth to form a single layer of fibre glass.
Once it has dried, pop it free from the mould, give a light sand with coarse sand paper the inside and lay layers of cloth and resin to the inside to build your layers up for strength.
Hope this helps