Sharpening the top edge enables the shovel's back to lie flat on the surface, getting all the material into it (e.g., mulch sitting on a concrete drive, or gravel in the bed of your truck).
Sharpen the inside edge using an angle grinder with a 120 grit flap disc. A metal grinding wheel may be needed for shaping the edge if it is in bad shape.
||If the original angle is different, it is best to match that.|
Be sure to remove all caked-on dirt. A wire brush or putty knife can be useful for this. A well-kept tool will last your lifetime, and will still be usable by your children (and maybe your grandkids).
Sap can be removed using a solvent. Acetone works well, but be sure to wear protective clothing as this is not kind to your body, and be sure the area is well ventilated.
After using any solvent, be sure to apply a thin coat of camellia oil to the tool. I like the spray bottle of camellia oil sold by Tools for Working Wood.
Some advocate using boiled linseed oil (BLO), but BLO often has heavy metals or other bad chemicals added for drying agents, and these are not good for you to handle, nor would they be good for the plants on which you might use this tool.
If the tool was exposed to any diseased plants or soil which is infected with pests, give it a quick wash in diluted bleach (1 part bleach to 8 parts water), and then rinse with plain water. Be sure to dry afterwards, and apply a thin coat of camellia oil.
Gardening tools often get stored for a while after sharpening, so it is recommended to oil the sharpened surface with camellia oil. (Indeed, all unpainted surfaces would benefit from this.)
Petroleum jelly is another option.