Lines of object behind showing through

[Buckyman]

Hi
This happens fairly often with the kind of drawing I do
In this case, I drew aluminum flashing (.03" thick) on a wood deck
How do I stop the geometry of the deck from showing through?
It happens on the screen and when I export PNG, but not exporting PDF
Thanks!

[Box]

Have a look here.
https://sketchucation.com/resources/tutorials/beginner/315-edges-bleeding-through-geometry