hi Rick;
i ran into the same problem this weekend on the inside of the deck on my spring run. my solution was to drill a 1/16 hole in each end of the bubble with a jewelers drill bit holder (kind of like a pencil) and fill the void with epoxy from a syringe, without the needle. the hole at the opposite end of the bubble allows air to escape as the epoxy takes it's place. it worked very well and was a hell of a lot easier than sanding and patching. give it a try, if it doesn't work for you you can still do the sand and patch thing later.
hope this helps;
daren.....