Russell, if you are gonna buy a heat gun get something like a Wagner. About $35 if I remember correctly. The cheap Harbor Freight guns have been known to burn out prematurely.
Also, if you shellac the back before steaming or boiling you can prevent the drying cracks while it's drying. I use Bullseye spray shellac. About $8 to $10. Canned shellac has a shelf life but not the spray that I have noticed. Shellac flakes last indefinitely but you have to mix what you plan to use. I keep spray shellac on my work bench all the time. Never know when you'll need it. I spray the back of every bow I chase a ring on, no matter how long it's been drying. I've had staves that have dried for 10 or more years check when I exposed a new back ring.