Hmm. Not sure what your process is, but I don't get seams when I texture anything in ZB unless I want seams. 

You can hide portions of the model as needed, or turn it on an angle so you don't get that streaking. Avoid painting or spotlighting too close to the edge in one pass and make sure you don't have double sided turned on or the color might pass through to oposite geometry in places if you aren't careful. You can also soften and sharpen the mask to blend seams. It all takes practice and patience. Texturing is slow if you want it to come out right. 

There used to be a program called Deep Paint, but it was expensive, and I'm not sure its even still available. 

You could also try this: http://www.allegorithmic.com/products/substance-painter that Snarlygribbly linked to the other day. It has a free demo, maybe it will work better for you. I haven't tried it so no clue how it compares to zb.

 

~Shane