Before we move on to mannequin retouching, let me say a few words about transparent clothes on models. If something is transparent and you can see the skin underneath the material, it's not a problem at all. It doesn't require any retouching. The problem only arises when you can see something that shouldn't be there. Then you have to Photoshop it out of the existence.
Oh, wow! The model is wearing her own panties under the transparent panties. How thoughtful. Really, the image can't go to the website like this. Customers won't be happy when they see that. So you have two options: the first one is to reject the image so that the item will be reshot properly. There are special hygienic pads for that, you can't really shoot transparent panties if the model is wearing her own panties. It works with briefs, but not if they are made of mesh. But there's still the second option: brace yourself and retouch the image, removing the other panties completely. I'll show you how to do it.
First of all, I want to tell you this: it is not only possible with the Frequency Separation. There's enough texture for Clone Stamping, but you'll have to be very careful and accurate. So why bother if the texture allows us to split the frequencies? I'll do the splitting with the Radius of 3, because the image is relatively small, just 2000 pixels high. Then I'll go to the low frequency layer and use a normal brush to replace the small panties color with skin color. It's not really easy, as the tone is not even, and there's also a shadow which is supposed to stay. Pay attention, that there's also a crooked label waiting to be removed. There will be spots when I finish, so after that it's worth to use the Mixer Brush to make the surface even. The panties will hide on the high frequency layer and they will not be affected with what I'm doing. If they obscure the view, you can switch the top layer off to be able to see the color layer better. I don't usually bother to do it, but it can be helpful.
As soon as I'm satisfied with the result, I'll switch to the high frequency layer and use the Clone Stamp tool to remove the panties completely. It is a bit tricky with the source area being so small, and the destination area being so large, but in the frequency separation mode it's quite possible. Just be careful and watch where you're stamping. In the end, make sure you've done all you could on the separated image before flattening all the layers in one. There's still a bit of stamping left in the label area, as there's no shadow and it looks weird. But it doesn't take long, not after all the work we've already done with the image. And it looks much better than the original! But would you really like to do this every time you get to retouch underwear?
Labels can be a pain, especially under transparent clothes. It's very reasonable not to leave them under mesh if it's possible. Here, in this image, a stylist did a very good thing: they pulled the label up so it ended up on the skin, not under the transparent material. It's easier to remove this way. Before you try anything else, try this: select the label with some space around it by using the Lasso, and then press Backspace to access the Fill window. Pick the "Content-aware" as your contents and press OK. Photoshop will try to think of something to fill the selected area with. And sometimes the result will be surprisingly good! Like this time, when I only need a couple of strokes with the Clone Stamp tool to finish it.
Let's try it one more time. Same as before, select the problem area with the Lasso making sure you leave some space around. Fill it with the Content-Aware Fill and there you go! Not perfect, but not bad either, and if you wanted to remove it manually, you would have to spend much more time than that.
Finally, there might be a situation like this: some garment is see-through, and a stylist or a photographer come to you and say: "This is not supposed to be transparent! It's only like this because of the studio light, can you make it less transparent?" Well, of course! Why not! The deal is that you can see some light dots through the dark material. It means that you can use the Brush in Darken mode. Pick a color that is not darker than the item in general and paint over the skin seeing through. Make sure it's not darker than the item, or you'll kill its texture. See, it's not a big deal.
In mannequin retouching, transparent clothes are a major pain. That's because you're supposed to remove the mannequin somehow without removing the clothes. Fortunately, in model retouching, you don't have to remove models, and this fact makes transparent clothes somewhat bearable. It's only a problem when there's something under them which is not supposed to be there, like a label.