Read about painting the trunk of small trees with diluted white latex paint. What good does it do for fruit trees? They mention sunscald protection, but did not go into detail.

If you live where it gets very hot during the summer, the sun will damage the trunk on seeding trees. Painting seems to reflect the heat off the trunk. If you mix a bit of lime-sulphur into the paint, it helps repel various insects, including borers from the trunks.

Paint makes it easy to see if any pests are trying to dig into the bark. Paint makes the trees very ugly however.

I live where summers get very hot, and find the paint helps trees do better the first 5 years or until they have enough greenery to shade the trunks.

up here you need to paint the tree trunk (or wrap with white plastic) for the opposite reason.

In the spring time the sun warms up the sap, before it is really spring. Then at night it goes back to a hard freeze, and that sap expands and pops the trunk. If it is bad enough, it can kill the tree.

So here, if you have a tree with dark bark on the trunk, you need to cover it come fall and uncover in the spring. The people who paint, use a diluted paint, so it wears off by spring time.