The idea is to apply a surface sealant that you can sand really smooth (I believe they call this process fairing). I use a mixture of epoxy resin and glass microspheres for the job. Tinted with a little black to make it a bit darker and easier for me to see.
Top side has a coat drying and once it's set, smear a bit on the bottom
Before all that, I layed down a bit of glass reinforcement to the underside and along the features of the tank's shoulder because it's kind of delicate with just the foam. Don't want this thing fracturing when building the mold pieces