I've posted in "Ask the Expert" and pose this question(s) here as well.
Big oak tree was recently treated for borers and deep root fed, dead wood branches removed. No more evidence of borers.

There is a large area of bark missing at base of the tree because of decay underneath the bark. I have removed some additional bark in hopes of finding undecayed wood I could tar seal to help the tree get well. I now have about 50% of the bark removed at the base and extending up to my height with "hollowness" under the remaining bark almost completely around the tree circumference indicating more decay. (termites) Am I causing more harm by removing the bark to get to the decay? I am trying to save this big beauty. Am I heading in the right direction? Help! As a post script:
It seems it is impossible to find a tree tar made specifically for trees in any quantity. Will any good quality roofing tar that will cover and stay flexible work?(see my "Ask the Expert" post)

Reply to post by Richard Law, on November 03, 2000 at 11:37:11:

Q: Am I causing more harm by removing the bark to get to the decay?
A: No, if the wood under the bark is dead, it is rather good.

Q: Will any good quality roofing tar that will cover and stay flexible work?
A: No, it could even cause more problems.

You can't treat the tree, but you can make it healthier by phytohormons/bio stimulaters, improoving the soil and other activities like this. It can be alive plenty of time if you do that. To know what exactly to do, ask a good treedoctor residing near your place.