I have used Formula-U a lot over the years. In my experience, the older the paint gets before it is used, the longer it takes to cure. I once had a cowling that took a week and a half to cure where it wasn't sticky. Once cured, it is great stuff. I have seen it degrade with constant exposure to fuel and oil to where it will wrinkle up and turn into a mess. As long as you can wipe it down at the end of a flying session, you will have no problem with it but I think this makes it undesirable for fuel proofing.