Jeff, you can use this on finished bows...but...a lot depends on the wood used and the degree of twist and damage to the wood itself from being shoot in this condition over the years. What wood is your bow? Osage reacts well heat straightening whereas some white woods might not be as effective.
Allen, heat treating seems to work best on white woods that generally are not compression strong(like hickory)and woods that aren't too oily. It doesn't work as well on osage but osage is usually strong enough in compression and doesn't need the extra strength. Hickory, maple, elm ash and other white woods seem to benefit quite a bit with heat treating. I am new to the heat treating so I'm experimenting as I go, now.So far the results are promising.
Marc St Louis wrote a chapter in the newest Trad Bowyers Bible(TBBIV) that explains the whole process and the benefits and drawbacks of heat treating. Marc has made some fantastic highly stressed static recurve elm self bows that are shooting hunting weight arrows over 190fps.
Pat