HOw do you export textures that you have polypainted onto models in obj?
You can export as either OBJ or STL. With OBJ, you will need to import into a program that can read the custom polypaint tag, such as ZBrush or XNormal. STL should be OK with any program that can import it.
To export OBJ:
Press Tool>Export and save the file.
To export STL you use the 3DPrint Hub in the Zplugin palette:
Press Zplugin>3DPrint Hub>Export to STL. Make sure Polypaint is on in the Export Options.