We use tannin from time to time at the beginning of the ferment but it can be added later. You should see the manufacturer's specifications for the range. It often helps otherwise unremarkable juice with mouthfeel and depth. If after ferment, you can add incrementally to your taste. Good luck!
Extended exposure to air can allow contaminants like particularly surface yeasts to infect the cider which can impact flavor. When you are racking a fermented cider that is aging, you should make sure to fix an airlock on it again as soon as you can and keep the airlock functional. Better practice is to use an inert gas to expel any air in the top of the vessel but often residual fermentation can cause that air to be expelled with co2. Good luck!
