You do not have to paint or seal driftwood that you put in your aquarium. The wood will release tannins into the water and lower your
pH, which is good for many species of fish. It will give your water a slight tea color but boiling the wood first and using carbon in your filter will help with that.
I have read not to use cedar in an aquarium. Stick to hardwood like oak or maple.
I’ve had driftwood in my aquarium that I found in the water but pulled it out because after several months it was still floating. I guess I’m a little impatient. The stuff I found was really pretty, not to mention free but I had already bought some online so I just use that.
Be sure to boil it so you don't add bacteria to your tank. You can pour boiling water over it if it's to big for the pot or put one end of the wood in the pot and then flip it over to boil the other end. I had a really big piece that I put in the laundry sink and ran really hot water over it for a while because it was so big. Of course, it’s best boil it for quite a while.