Hi John
If it’s done correctly it won’t damage the varnish
But the secret is applying enough heat to take the bend out without blistering the finish.
Steam is not the best way to do it as it adds moisture at the same time what you need is dry heat the best from a spirit lamp.
Hold the rod depending on the size of the flame a few inches above the heat and continuously turn the rod applying the heat as you go to the area.
The right amount of heat is just sufficient so you cannot hold the section to your lips. So you need to keep testing it. Once you get the rod to that temperature counter bend the rod slightly and hold it for a few minutes.
Let the rod cool down and sight down the shaft.
If all is well its job done if not you may have to go thought it all again.
It’s easy for someone who knows what there doing but not so for a guy who as never done this type of thing before.
Its up to you
Gary
Nicho Bamboo Rod Co