The white line problem happens when you try to inpaint and outpaint at the same time, or when you use both tools at once. Try to avoid doing that. If you're outpainting, do that first, a step or two at a time, and then do your inpainting.

But sometimes it can't be avoided, or maybe you just prefer to do manual touch-up. Here's how:

zoom in to the desired level of detail,

use the inpaint tool with a small brush size to select the imperfections;

set Attention to 0, and run an IMG>IMG generation.