So I flipped my mesh under display setting and did a bit sculpting.
However, as I move to polypainting, when I tried to fill the cloth with blue color, the opposite side get filled. I tried to filp the display back, then the mesh looked broken…
I tried to enable/disable double, but it doesn’t do anything…
So what shall I do in this situation? Many thanks everyone! 
Hello Ophelia,
Am I understanding correctly that you flipped the normals with the Flip button in Tool> Display Properties?
This would not be an ideal way to hide geometry to get at an underlying surface. Those polygons are still present and editable in your scene, you just wouldn’t be able to see how you might be affecting them. Not to mention flipped normals introduce other concerns to your geometry. It’s generally not an operation you would perform casually.
A far better way is to actually hide geometry if it’s in your way. While hidden, the geometry won’t be affected by your brushing at all. The visibility functions can be found here:
http://docs.pixologic.com/user-guide/3d-modeling/modeling-basics/mesh-visibility/
Note that this becomes much faster if you actually have well-defined polygroups.
These functions would be mainly useful for geometry that occupies the same subtool. If you don’t need to be able to sculpt on a piece of geometry at the same time as another, for instance if it was a garment separate from the actual body, a better way might be to simply split the geometry into its own subtool. Then you can simply toggle the visibility of the subtool off, or work in solo mode, so that only the active subtool is visible.
As to your issue here, it’s very difficult to work out what’s going on from the picture. My guess is that you had a garment with a degree of thickness, with an inner and outer surface facing in opposite directions. When you flipped the normals the outer surface was turned inward, and the inner surface was now facing outward, and would have been deformed by your brush strokes if any sculpting was done. It looks as if some of that inner surface is poking out through the surface of the body, while some of it is still inside the body. In short, it’s probably a bit of a mess.
Is the top it’s own subtool? If not, it would be much easier to work out what’s going on if you could actually split that garment into it’s own subtool, so you could work on it independently of the other geometry in the subtool.
You can use the visibility functions I linked above to hide the geometry, and use the Subtool> Split > Split hidden function to split it into a separate subtool. You could also use the Split to Parts option to split all the garments that are separate meshes and not connected to the body into their own subtool.
Once you’ve isolated the garment, make certain all the geometry is un-hidden with the shortcut in the link above. Then try to flip all the normals back in the correct direction. Please post a screenshot of the tank top, and only the tank top, with the normals facing the right way, and we’ll see what can be done.
Good luck!
Hello Spydel!
Many thanks for the helpful and detailed explaination! 
I’ve fixed my problem now, thanks to you! 
Sorry to reply you so late! I had a busy time last month.
Yeah, like you said I flipped the normals before, and messed up the inner and outer geometry, then did a lot of changes. 
Exactly, as you said hide/spilt geometry works like a charm! I redo my top and add some textures, and it all worked!
Also, I figured out a quick fix in the meantime, i.e. hide the inner part of tank top, just extrude the face that is fine out, delete hidden. Then do some deformation to smooth the new inner part of the tank top, and add subdivisions.
A big thanks again!