I am using this stuff to seal / fuel proof nd engine compartment.
I applied it 26 hours ago and it isnt really hard to the touch. I can scrape some of it off with my fingernail. Does it take longer than this to harden up? Or, is this about what I am going to get?
I am considering thinning some epoxy and brushing it over the paint.

What do you think?

I don't know the answer to your question. But, I sure hope someone does. I am covering a 4*120 now and am going to do the same thing. Although if it doesn't work out for you I might switch and use Rustoleum.

I have used the spray cans and I have sprayed the bottled stuff thinned out for spraying.

it does dry slowly. It really takes months to harden nicely

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.