In these circumstances we recommend fixing levelled battens to the wall first. If this is not possible, please be aware that the wall may not be flat, and you may need to use washers to both space the Buttons off the wall for levelling, and to keep them parallel to the panel: although there is a degree of flexibility and tolerance in Button-fix, misalignment of the Buttons and Fixes may cause binding, and difficulties in engaging or removing the panels.