Ask Mr. Smarty Plants

Most trees will heal damage to their bark over time - particularly if the damage is less than 50% of the distance around the trunk. The recommendation by arborists is to not apply tree paint as this just causes a location for moisture and pests to congregate.

What is recommended is to "trace" the bark and cut away the damaged tree bark so that there is a nice smooth, oval edge to the bark that will heal fastest.You will need a sharp strong knife or a chisel and hammer for this task. Don't cut deeper than the bark layer. If the damage is more than 50% of the tree's diameter and the trees are valuable to your property, consult an arborist for advice.

Texas AgriLife Extension has a Tree Care Kit article online by Wayne Clatterbuck that has advice and a picture about Tree wounds.

Here's what he writes: Tree wounds often appear ragged where the bark is torn during the injury. This is common during branch breakage and when the trunk of the tree has been scraped. To repair this type of damage, cut off any ragged bark edges with a sharp knife. Take care not to remove any healthy bark and expose more live tissue than necessary. If possible, the wound should be shaped like an elongated oval, with the long axis running vertically along the trunk or limb. All bark around the wound should be tight.