From what I understand, spackle and similar products are usually used as finishing materials for PM, to smooth over the lumps.  One advantage of it is that it can sanded afterwards.  Some people spackle and sand, spackle and sand, for a smooth finish surface.

The point of PM (as I see it  ) is lightness with relatively good strength.  If you're getting too much into heavy finishing products (to the point where the spackle/etc is the main ingredient), maybe a re-think of the project should be made.  PM may not be the best material for an absolutely smooth finish for your particular project.

Another possibility may be using paper pulp for finishing.  Some of the PMers swear by a pulp made from toilet tissue and adhesive, painted on the basic form.

Some crafters mix calcium carbonate (also called whiting or ground chalk) with their adhesive and spread or paint it on the surface.  The chalk helps fill the crevices.  Sanding between coats may help, too.

Using PM with negative molds can be quite useful.  It does tend to shrink, depending on what combination you're using, but that could be a benefit. 

But if the perfection of the initial finish straight from the mold is a real issue and PM doesn't do the job (try it first), we could be back to the question of how appropriate PM would be for the project at all.

Experimenting is probably the best way to find the results you want.  Nothing teaches you what works and what doesn't better than hands0on experience.  You could try several combinations of materials to see what works best:  PM strips with adhesive; PM pulp with adhesive; a combination of the two; wet paper strips with no adhesive for the first layer inside a mold, followed by strips moistened with adhesive; pulp mixed with adhesive/chalk combination... there are many combinations.  And even for one person, not every combination is going to be appropriate for every project they want to do.

Don't forget the mold release if the mold isn't plastic!

Good luck!  Let us know how it goes.

Sue

I endorse all that Sue (Catperson miaow) has said.  Just one further detail.  Elsewhere, I learnt that if you lay damp non adhesive paper into the negative mould, once you have a complete layer, brush PVA thinly over it before laying the next layers.   When it is released, you might have a few small corners to paste down.
DavidO
(dopapier)

... I've been using a plaster cast for my masks, I will try what CatPerson's says- toilet papers... I'm getting tired of the texture I'm getting... 'been using hand mashed pulp... try petroleum jelly on the negative mould, make sure to apply it evenly throughout the surface before the pulp... I usually leave it for a day to dry, once it shrinks a bit it's ready to be removed from the cast... some will need retouching- the cracks, holes, and chips...
... if your mask has intricate details, plaster cast may not work for you... a rubber resin mould will... a friend of mine knows how to make one... I'll ask her to teach me when she's not busy... well... good luck... keep us posted...