If you want to paint over the edges of those maps, that's what software like Blacksmith3D, Substance, Zbrush, etc are for. I don't think you're going to get that right in a 2D graphics program.

If you don't need to go to the edge, you can use the UV maps / templates for the model to know where the edges are, or what I usually do is load an existing map as a layer and then paint over it in another layer. That gives you pretty good references for where you are working. Then hide that layer before you export the result.