If only using half a gallon, can you store the remainder, or do you have to use it all at once?

Flex Seal Liquid can be reused. Just be sure that as soon as you pour out the amount you wish to use that you seal the can back up tightly. As soon as air comes into contact with the product it begins to cure. If a skin does form on the top of the liquid when you go to reuse it, be sure to remove it first before using. Send any other questions our way. Have a great day!