Faceups on polymer clay?

I don’t know if you have Liquitex in the UK, but the Liquitex matte varnish applied in thin layers with a makeup sponge is all I’ve ever used and I like how it turns out.
I’ve done a couple faceups on polymer clay heads, and I didn’t have any trouble just using normal faceup techniques. I could be wrong, but I guess it’s because I was putting down sealant to start just like on a resin head, so technically you’re putting faceup media on the sealant, not the clay. Your results are going to be better if the head is finely sanded. However, I don’t think you could wipe the faceup off the same as you’d wipe a resin head.

 

From my experience, the faceup techniques don't really differ that much - you seal, you paint, you seal again. As you want to cast it afterwards, it'd be good to cover a test piece in sealant and see how well it wipes off and if the polymer clay reacts to anything. It can be quite a bit slicker on the surface than resin so you'll need to make sure the sealant creates enough "tooth" for pastels and watercolour pencils to stick on. Citadel Munitorum Varnish and Liquitex Matte Varnish (not the spray! The one like @Bexalex described) should be fairly easy to get and are tried and tested for faceups.

 

The face up can probably be made using the regular materials (allthough, tests are always a good idea) as long as the finish of the surface is good enough. But it can cause problems later.
If you want to use the same piece as the master for moldmaking later I would recomend against doing a face up now. Not so much because of the sealent, but because every additional material you use (every colour of pastel, every paint etc.) introduces a risk for a bad reaction with the primer you use to later prepare the part for casting as well as with the silicone during moldmaking. To be safe, every single one of those materials need to be tested with the moldmaking materials and process you will be using and that sounds like and absolute massive amount of work for something like this.

And, not related to the face up, but there is a problem with putting a polymer clay head on a strung doll. Polymer clay is not very strong and can crack under the constant pressure. You may be able to increase you chances by stringing the body very loosly, but it's still a risk. If the head is basicly ready for casting I am guessing you have spent a good number of hours on it allready? Is it worth risking all that for a single ocation?

 

I’ve no idea if it’s a similar material as I don’t really know much about polymer clays however I’d be very very cautious of using Liquitex varnishes or mediums on clays.
A friend had a bobble-head of himself made (well his girlfriend ordered it from one of the many places that do these personalised bobble-heads) and he wanted me to try and paint it up a bit more to look more like him. Even after trying to research what was safe and unsafe to use it has still been a nightmare, acrylics are not drying, especially any thinned with Liquitex medium……they remain tacky after 7 months.
Liquitex varnishes I’ve also had issues with on soft materials such as vinyl dolls….again it either remains tacky or dries initially then becomes tacky and unstable several months down the line. I think I remember someone in another part of the forum mentioning that Liquitex’s mediums and varnishes are hydroscopic (?) and attract moisture in humid or damp environments. Wish I had read that snippet of advice before I used them, would have saved a lot of heartache.

 

Thank you for the good advice @Lillith and @Blodeuwedd. I have similar concerns about molding a head in sculpy. I planned to make a cast and use some kind of resin but I haven't decided what kind yet. I'm most familiar with the clear stuff that comes in two quarts from Blick's.