Don't forget Blender do basic painting (with some parameters like transparency , you have alos blurr tool ) in 3D directly on the mesh and also if you want in the bottom window in the diffuse map directly if you want

I use that mainly ,i do a rought diffuse map, than opne it on Gimp2 , work it
with all great tools , than just do refresh under blender from times to times to see if results in 3D are what i need.

Blender is free Gimp 2 also