Faceups How to layer pastels?

You do need to seal between layers. That is what makes them layers after all, otherwise it would just be one thick layer.

 

Yes.

Okay, longer explanation:
the surface of your doll is not fully smooth, especially after you have added sealant. It is a little porous, and that allows your pigment to stick onto something. It goes into the "deeper" parts of the surface.
Now once this porous surface is saturated with pigment, you cannot add on more. There is nowhere for the pigment to go to, nothing for it to stick onto. If you keep adding pigments now, they will just loosely sit on the surface and keep being pushed around. If you add sealant though, you basically add a fresh layer of porous surface on top again. Now the pigments have little pockets again to fall into.
I made a crappy doodle to show the concept:


So if you want to build up color, you need to do it in several layers. Personally I usually work like this:
- sealant base layer
- light base blushing eyebrows first, then light overall blushing to map out things
- sealant
- blush enhancing layer
- sealant
- if needed, another blush layer to enhance things further, else I do the first acrylic layer
- sealant
- enhance acrylic details, add some blushing still here and there
- sealant
- optional in case anything still needs enhancement or I want to add shimmer
- semi-gloss finish sealant layer (for skin effect)
- glossing lips/tear ducts

So I usually do six layers of sealant overall, maybe seven if need be

 

That's what I hated about working with pastels when I was bay in college. You have to seal every few steps, even when you are working on paper. Otherwise you'll get a cakey mess that will smudge. They are wonderful for shading, and I still enjoy doing so with them on larger areas like full body blushing, but will probably switch to acrylic mostly for face ups, since there's absolutely no need to seal between layers because paint sticks to paint, unlike powder -- you can pile on powder, but have to seal everything all the time. Otherwise , it's just dust on top of dust. (:

 

Others have already pointed it out regarding sealing to avoid cakey-ness, so I won't go further into that. Another minor thing to note is that pastels, by their very powdery nature, need multiple layers to "build up" color. Don't be tempted to put in a huge concentrated smudge of pigment in one go -- it's a waste, and will probably end up a splotchy mess anyway. Usually it takes me three pastel layers before I am satisfied with the intensity. And sometimes, it may even take WAY more, such as with the fiery gradient on my girl's fantasy faceup.

 

It's a good idea to seal EVERY thin layer you're happy with. And there's a rule with pastels - when you think you just need one more layer it means you should have stopped a layer ago, It's a bit of a joke, but it kinda works. And if you're new to pastels you should get to know how different colors you work with behave when sealed. Sometimes colors can become more prominent and vice versa paler or even change thier shade a bit when sprayed with MSC.