Sometimes a postage stamp affixed to a cover doesn't appear to be worth very much.
If it is taken off the cover and the water mark is examined it may turn out to be more valuable.
My question is:
a. is it acceptable to take a stamp off the cover?
b. can the stamp be re-affixed after examination (if so what adhesive should one use)?

I have recently taken stamps off of covers, ensuring that I made good quality scans of the cover/s with the affixed stamp/s beforehand.