The solution for the line down the middle is easy: Do not add another layer before having thouroughly sanded down the line in the previous layer. Even if you go back to the edge on the sides, do not stop until that line has disapeared.
Depending on the grit used, it might take time. I roughly sand for maybe 10mins per edge with a 600 gritt with just a light press.
What I noticed though after running some tests is the thicker the layer, the deeper the "valley" in the middle.
And I do not believe it is a leather absorption issue as the first layer usually does not develop and deep line, like 30secs of sanding and you are good to go. The second and third layers is where it is all at.
For me, I found that my best edge work comes from 1st and 2nd thick coats (Or 1 primer + 1 thick coat) with a medium third. Work the best for me.
Hey man
The solution for the line down the middle is easy: Do not add another layer before having thouroughly sanded down the line in the previous layer. Even if you go back to the edge on the sides, do not stop until that line has disapeared.
Depending on the grit used, it might take time. I roughly sand for maybe 10mins per edge with a 600 gritt with just a light press.