Since this aircraft has an EPO airframe and we are using gasoline (petrol, benzine) as a fuel, I figured I should see just what effect this fuel has on the foam. The good news: gasoline does not disolve EPO. The bad news, it does weaken it significantly. More bad news: water-based polyurethane doesn't protect the foam against gasoline damage.
Now I will try some other options including acrylic paints and if that fails, I'll just use a two-part epoxy layer on the inside.
Stay tuned, more to come!