How to prepare salsify (schwarzwurzeln) to minimise sap?
Rinse the salsify first before using, that will help.
Use a sharp peeler. Ive found that dull peelers create a lot more sap.

as soon as you have peeled it, drop the salsify into acidulated water ( water + lemon juice).  The acids help prevent the color change that occurs.  

Also wipe up the area with a damp cloth as you work.  This will keep things clean and prevent your hands and everything you touch getting tacky.