Bora-Care can be applied to any raw wood that is not painted, stained or sealed so yes it can be brushed on to the underside of the flooring. It will only prevent termites from entering the wood that it is applied to and will penetrate up to 4 inches with one application. It should be applied until the wood is just wet but not dripping. Most of the time termites would have to eat through the sub-floor to get into the hardwood floors above, but there is no 100% guarantee that the termites will enter this way. Treating the sub-floor would also ensure the termites do not get into the sub-floor as well. If you want the hardwood floors better protected, you should treat all exposed wood also as long as they are unfinished. Please see page 10 of the Bora-Care label. Table A on page 10 will tell you how many lineal feet of board the bora-care will treat based on thickness. The tabel tells you how many lineal feet a DILUTED gallon will treat
The Bora-Care is currently only available in the gallon size.
You want the siding to be dry when you are applying Boracare per the label. It needs to be able to soak in to the dry wood.