I would like to paint on my 3D object in Substance Painter and have UV unwrapped it as in Picture 1. However, when exported to Substance Painter, it's showing strange lines. How do I remove them and am I UV unwrapping this correctly? Thanks!
The problem is that there are there are overlapping areas in your UV map. Thus, when you paint on one area in the texture, it affects the other portions of the UV map that overlap that same point. You'll probably want to try using a different UV projection for the model. Try out several options until you get a nice UV map that doesn't overlap. Just note that some projections are view dependent. So try going into a view that makes sense (like front view) before unwrapping. You can also try electing edges in problematic areas and selecting Ctr+e -> Mark Seam so that the UV map disconnects along that edge. Once you got something that looks pretty good, you can head over to the "Texture Paint" mode to get a basic idea of how good the UV map will work before importing it into Substance Painter.
Hope that helps!
Use Smart UV Project in Blender to ensure you don't have any overlapping areas. Manual unwrapping requires careful placement of seams (and is recommended for many complex objects, say, a human face), but for this, U -> Smart UV Project will get rid of the issue, and additionally not have stretching, for example.