My mulberry tree was leaking sap earlier and it left ugly black stains down the trunk. The tree has deeply furrowed bark. Can I remove this?
First, I can't stress this enough, before considering to remove the sap from the trunk, it would be best to take the time to identify the reason for the leaking sap ie Your tree may very well have bigger problems than just sap stains on the trunk...
Second, I would be very wary of removing sap from a tree trunk, doing so can lead to inflicting even more damage on the tree.
Third, keep in mind it will be almost impossible to remove all the sap from the trunk .
Keeping the chisel as near parallel to the trunk as you can, very! carefully slice, pry off the sap. Whatever you do, do not let the chisel cut into the bark, or allow the sap to pull away the bark from the trunk. If the sap is really hard, use a hammer to very! carefully insert the chisel into the sap.
Once you've "very! carefully" finished removing as much of the sap as you can from the trunk with the chisel, take a clean (with rubbing alcohol or similar) woodworkers card scraper or decorators/painters hand scraper and very! carefully (this time keeping the tool perpendicular to the trunk) scrape off as much of the remaining sap as you can without damaging the bark.
I recommend stopping there, don't try to remove anymore sap.
If you felt compelled to try and remove even more sap, the following may work (disclaimer, I've never tried it).
Mix up some eco-friendly "degreasing" cleaning solution (read the label to ensure it is safe to use around plants, etc), then apply directly to the sap via a (new) stiff bristle cleaning brush. Allow the solution to "work into" the sap as per the instructions on the label. Then thoroughly rinse off with cold water.
Prune the tree at the correct time of the year for that type of tree to avoid the stain from a leaking wound.
Over several years the stain does go away as the wound heals closing the leak. It's not a sick tree, it's just healing the wound!