Tan sanding issue - Why??

The short answer is that when you sand you change the texture of the surface and with that how it reflects light. If you can manage to match the surface to the unsanded it will look like the same colour again, but thet is not always easy and depends on a whole range on factors.
The more saturated the colour the more drastic the effect is.

The extra cost depends on several other thing as well, but one factor is that because of the dificulty in getting a nice finish you'd typiccally want to use fresh mold for tans while you can still get reasonably good cast in paler colours in a slightly worn mold.

If you want more details, try do a few searches. the topic has been coming up quite a lot.

 

Lilith is correct.

The thing is, sanding leaves tiny scratches on the surface. They can be extremely small and barely visible to the naked eye, but they are still there.
You see that a lot among resin casting circles, like for jewelry or dice or even furniture. People bemoan that once they start removing seams, other flaws or similar, their cast appears lighter or cloudy. You need to really, really polish darker colors to make them appear the correct color. Of course that sucks a bit when you don't want a shiny finish. But it's either that, or a less deep color.

That's also why a lot of Chinese companies send their dolls looking so glossy and shiny After removing seams they completely buffed them to get rid of any scratches and uneven color.

Also the reason why darker colors are more expensive is because:
- dye costs money (well d'uh)
- achieving an even result across so many different parts is difficult
- the more dye you add, the more likely it is for there to be some funky results during curing
- sanding and polishing the cast takes longer too, since you need to sand it all over to keep the even color
It's way easier to cast a completely white doll and have it all be the same color, with good quality throughout vs darker casts. And afterwards the finishing takes more time too.
So you not only have a lot of waste and struggle more, the finishing also takes extra time.

 

I read that in the early days of resin BJDs tan dolls were in fact molded with only tan on the outside and a lighter color for the core. These days the resin is uniformly colored for tan and fantasy colored dolls, but as others have said, sanding leaves scratches that make the sanded spot look lighter. Progressively sanding with finer grades of sandpaper can return the resin to its original color, but doing so is time consuming and labor intensive.

I've sanded seams on a couple of my artist cast dolls, and I can see why many companies don't. It's pretty tedious.

 

I have sanded a jet black ResinSoul Xue before, and at first, it does look lighter, but once I got it polished enough, which did take some work and the proper sand paper, it did end up matching again. I think most companies warn against it because it's so much work, and without the proper tools, the doll probably will come out looking worse than the seams do. I can understand them not wanting to get blamed for someone ruining their doll.

About casting dark resin, I haven't done it myself, but I remember when Jie doll stopped offering darker skin tones (ages ago), it was because it's so hard to cast. The color can come out uneven, it can snowflake, marble. If I remember right, they said for every ten pieces of resin they cast, they had to throw away at least one piece. That's pieces, not dolls. I'm not sure if that is normal for most companies, but if so, that gets really expensive, really fast.

 

Here's how I understand it works:

Imagine holding a piece of smooth dark resin under a lamp. Most of it will appear dark, but you will get a specular highlight on top: a bright spot where the light from the lamp is bouncing directly into your eyes. The colour of the resin isn't different there. It just looks brighter because the surface there is at exactly the right angle between the lamp and your eyes.

If you now sand a patch of the surface that appears dark, you will create a whole bunch of tiny peaks and valleys. And on each of those tiny peaks and valleys, there's going to be a teeny tiny spot where the angle is going to be just right, and you're going to get a whole bunch of teeny tiny specular highlights. Your eye will see this collection of highlights as a 'light patch' because they are too small to distinguish from one another. But importantly, the colour of the resin itself hasn't changed.

If you now do the same thing with white resin, you will see the same effect, but it will be much less pronounced because the difference between how the highlights and the surrounding area look to your eye is much less.

And, as others have noted, you can make the light patches disappear provided you can get the entire surface to be consistently smooth (or rough) again.

 

^ What Lilith and Ara said.

Depending on how darkly pigmented the resin is, the sanding marks can be very visible or unsightly: some makers prefer to not sand the seams of these dolls to reduce the overall areas of discoloration. Additionally, you will get light marks appearing in areas of friction (joints) from regular handling of the doll.

Some heavily pigmented resins (tan, grey, etc) also have had issues with being more brittle than lightly pigmented resins and may be prone to breakage, especially on older dolls.

 

I am spoken to doll casting companies before and some actually said that it is possible to sand and not have color difference. Depends on how they mix the batch I guess. But I would test on an area first, preferably between the joints where it is not obvious.

 

That^^^. All of that. I have been modding dolls for a lonnng time and yes you can sand the darker resins but No I wouldn't recommend it-

unless you're extremely patient EXTREMELY thorough, and very good with graduating your grits of sandpaper smoothly, and maybe even going on to plastic polishes.

Now don't get offended yall, but my experience has been that most people are not all of these things at once, and will often rush sanding or skip it entirely, or do it partially and then get discouraged and stop when the work turns out to be very time consuming and not rewarding, or because they fear they have ruined their doll. Sanding is simple, but it can become complex when you're doing something this thorough.

 

Another thing to add to this: it's actually REALLY easy to over-sand something, too. We've all done it once, and I imagine most failed projects fall under this category. Unfortunately, sanding is really an acquired skill, so there's no other way to learn than practice! Just... maybe start with model kits, woodworking, or something else inexpensive before potentially ruining several hundred dollars of resin.

Seamlines aren't worth removing to begin with as far as I'm concerned; better to save sanding for additive and subtractive mods where you're going to paint over any potential flaws in the first place.