Have to disagree with you, Zwickey's (& Magnus) are the EASIEST head to sharpen, in particular the Deltas because of their straight edge.
They usually come slightly concave, some more than others, and that is dependent upon where the grinder was in its wear or adjustment schedule during the sharpening process. I straighten the edge so it is perfectly straight or flat to a true surface.
Always use a NEW (or new-ish) Mill Bastard file, that makes a HUGE difference. Maintain the integrity of your files and always cycle through older ones; retain those for shovels and axes.
I C-clamp the file on my bench with it sticking out into my shop, suspended. I run the head on a shaft, the length of the file pushing from tip to toe, parallel to the edge, maintaining the angle with just my hand. This takes some practice & skill. As others say, raise that burr, then remove it! I also hone mine on wet/dry paper on a marble slab used for sharpening plane blades and a paper wheel w/rouge on my grinder.
My arms are bare of hair right now...it blows off with the touch of a blade!
Kris