Depending on how hard or soft the cut objects are will usually determine how often my blades need sharpened... I will sharpen all at the start of spring growth, nice soft moist new grass, easy cutting... then late summer when things start drying out I will resharpen on my largest mower, dry grasses dull my blades faster...
I doubt that sap would stay on a fast moving blade very long... In the spring about the only thing on any of my blades of deck, is a little green grass stain, everywhere except the cutting edge... That stays polished pretty clean.... :smile:KennyV