Here is some info I found about TentSure (in quotes). Seems like it's doable to me, has anyone had success with this method?

"Typically, you will coat the underside of tent flies, the inside of tent floors and the inside of packs and dry bags.

You will know when it is time to replace factory polyurethane coatings because they tend to develop a foul odor, they may begin peeling or flaking away or you may notice a sticky or gummy feeling. When this happens, it is time to remove the old polyurethane coating by scrubbing with isopropyl alcohol. It is important that you remove the previous coating before applying the new coating of Tent Sure.

Tent Sure will not work on on silicone treated fabrics nor silnylon."