im making a model for an university assignature. it consists of a geology cross-section with realistic terrain in the top, and flat faces in the sides. I don't really know if the edges of the faces are completely parallel to z axis, because i made these by myself. I used the extrude tool plus the 0 button and shift + z to supposedly align all vertices extruded in a planar fashion (don't remember exactly how i made it sorry), then i erased the interior lines and vertices of the model keeping only the vertices and edges of the extremes, but when viewing the result, the edges were slightly oblique to the axis, so i moved manually the vertices to align were i thought they looked good.
Anyway, my problem is when i tried to texture paint it. I previously selected the face and unwrapped it, created a simply blank texture in paint.net and assigned it to the face. When i paint a straight line in the 2d view, the 3d model show an offset pattern of the lines as the first image illustrates. How i could fix this to make the painting smooth and intuitive? Additionally, it would be very nice if there exist some smart way to edit the image in a third party software, using the black line that delimits the selected face in the UV editing menu, like shown in the second image. That would make my job much easier when it comes to drawing the geological strata and layers for the planar cross-section.
Thanks in advance.