When I click hide all edges and lines, the black lines disappear in my model but when viewed from certain angles, the lines and edges shows very weakly in white-ish colour.
Those are internal faces bleeding through where they meet the outer face. It’s a graphics artifact. You can hide the faces on the tops and bottoms of the walls so they aren’t visible to bleed through.
Here’s an example. On the left one the edges are hidden. On the right the faces are also hidden.
And with the three objects separated. The green is my back face color.
In that case, assuming the face orientation is correct, edit the back face color and make it close to the same green as you’ve painted the fence.