My opinion about the spots, on the previous photos is...
Too much ink, can not dry well.
If you set the head to move only on the width of the sign, bidirectional, and the room where the printer is installed is quite cold, then you may experience this problem.
The sprayed ink can not dry before the head return for the next pass.
The solution is to set all the heaters at 50 degrees, set the head unidirectional, and spitting on the media.
This gives time for ink to dry, before the next pass...
It happened to me.