The scrub plane blade has an arc-profiled cutting edge, with more or less marked radius according to the job required. This profile makes it more difficult to sharpening by guides so many people prefer sharpening it by hand. 
Im not very good at this job, so I searched for sharpening guides available that could serve the purpose. The Record 161 is a guide which is easy to find on ebayUK for few pounds. It has only one central contact point (a rolling ball in the middle) and this permits to incline to the right and left allowing to follow the cutting edge profile easily.
The blade is held firmly in place by a clamp screw that allows unrestricted iron orientation, useful even if it was necessary to sharpen skewed blade. The dark side is that you have to ensure the correct blade position by hand.
For bigger curves the guide can be moved to the sides of the sharpening stone to allow for greater tilt, as seen in the short movie. The result was satisfactory with 35 ° bevel. To re-sharpen exactly to the same angle Ill use a simple self made jig.