Help understanding mediums used in faceups

If I recall correctly, its a bad idea to use oil-based materials because they will stain the doll. Water based materials are less likely to stain so they are considered safe.

 

Hi! So using watercolors is generally considered safe as they aren't oil based, and also, non oil pastels are also great, as are acrylic paints. Each medium has its pros and cons and it's really about what works best for you, just remember to use MSc between layers and on the base so you don't stain the doll!

Oh sorry I misread, the reason oil based products are bad for the doll is because it can stain the resin, which can be a problem if/when the faceup needs changing, also MSc needs time to dry, you don't want to do a faceup while the MSC or whatever base your using is drying down

 

Basically, oils and solvents are materials which have the potential to have a negative chemical reaction on the plastic surface of your doll. Negative chemical reactions are something you definitely want to avoid.
Now it can get confusing unless if maybe you were a chemist, who knows in depth exactly how every material in question has potential to react. Because most of us are not chemists you will find accounts on the forum of materials which are reasonably tested by the community over years to be safe and those which unfortunately were discovered first hand to be unsafe, and then many educated guesses about what poses higher and lower risks.
Oil based paint, oil pastels, nail polish, enamel paint, and permanent markers are some of the materials to definitely avoid at all costs.
Now, onto the whole colored pencils thing. Once apon a time I made my own educated guess and used a lot of regular prismacolor pencils and nothing bad ever happened because of it. So I have often come in and said that I personally think certain brands of colored pencils can be considered safe. I feel like nobody ever listens to me and don't really care to argue about it anymore but there is the perspective of one person, that I tested them a lot over a long time and they seem as safe as most other recommended materials.
The reason hobbyists have enjoyed watercolor pencils is because you can use them in a different way than as a dry drawing medium. Using colored pencils as a dry medium you may notice some troubles such as grainy lines and even accidentally scraping off sealant while trying to work. People tend to use watercolor pencils in combination with the water. I never enjoyed that, so I just went towards painting fine lines with watercolor (another thing that people were widely telling me was impossible while I was explaining how to do it, but now it seems like it's accepted). Gouache and acrylic are popular choices for the line work on doll faces, and I think in minority, watercolor.
If you are interested in learning faceups I think the best method for beginning is to read guides about recommend and unsafe materials, learn about the basic "why" of each side so you can make educated judgements when needed, and then experiment with different safe materials and techniques until you find what you personally enjoy.
Ah and about the sealant: you won't be working with it while it's wet. It's something where you spray it and really need it cured before you continue, so it's pretty much a moot point. The aerosol sealants are actually a terrifying mixture of chemicals and solvents and resin which intellectually I would question how it's not ruining the dolls, but it's been working out for all of us so far!
Additionally, the actual purpose of the sealant is not exactly to prevent staining from other materials, and it will often fail you in that respect if that's what you are relying on, however it can help a bit simply because it is a thin, clear barrier. The actual purpose of the sealant is to establish the ground upon which you can work as you cannot work properly on an unsealed resin doll. Think of it all like pastel painting if you are familiar with that, or as a watercolor painter, even the difference between using different types of paper such as cold press and hot press, or with dolls more like any watercolor paper vs. Yupo paper. Having a slightly toothy ground is what makes so much of the work on top possible. Then the sealant between and after layers simply acts as your fixatif like when pastel painting.
Hope you have fun!

 

Not all regular colored pencils are a bad idea- just oil based ones (like Polychromos). Prismacolor Premier and Verithin pencils are wax-based, though the Premier ones would be terrible for faceups because they're too soft. I used Verithin pencils in my early faceups, and some of those faceups were on my dolls for many years and did not stain them. The sealants used for faceups do offer some protection, especially after you've applied many layers. But you still need to be careful about things that have high staining potential, like oil-based materials (so oil-based pencils, oil paints, oil pastels, and human makeup are out of the question). I couldn't say with confidence that wax-based materials are 100% safe, but for the most part waxy stuff just doesn't suit faceups anyway. But I think if you want to use Verithins, you can. They do stay nice and sharp, but I will say that they are difficult to remove if you make a mistake- which is one advantage of watercolor pencils, which you can usually remove with a damp q-tip.

 

Just to add: it's not only out of safety, some things are just really not great when you try to paint tiny details on a rough surface with them
Waxy or oily pencils for example are a pest to handle, oil pastels would not allow for smooth blushing and so on. There is a reason why certain materials just have turned out to be THE best choice throughout the years, and it's a mixture of us learning what is safe AND yielding the best results.
Also in case you haven't seen it yet, check out the BJD FAQ for general BJD related information including customization tips:
THE BIG BJD FAQ
You should also look at a looot of faceups to see how certain effects are being achieved. I think that helps to understand best why certain tools are used the way they are.

Also the most commonly used sealants are not water based, they are solvent based (Mr. Super Clear or MSC has been the standard for years, same for similar sealants like Zoukeimura's spray, Topcoat or the varnish from Games Workshop). The airbrush ones tend to be water based, but even those don't activate watercolor pencils while sealing.

I'd argue that if you're good at painting small details, skip the pencils. Diluted paint will always yield a smoother, nicer looking result than a pencil simply because of how it interacts with the textured resin surface. Pencils always look a little grainy. They are mostly the tool of choice of beginners because they are easier to handle than brushes, but if you are used to brushes you should be fine.