Plastic covering works well, i.e. monokote, ultracote, etc. This is my method of choice, but sometimes the plane design does not easily allow this method. In such a case, I use thinned epoxy. Straight epoxy will be heavy. You really don't need that much epoxy to proof the surface. Cut the epoxy with MEK or acetone until it is about the viscosity of warm syrup, then brush the mixture on. Some varnish in spray cans also works, such as Deft clear wood finish. None of these methods add any significant reinforcement to the fire wall. Reinforcement requires structure, not a coating, unless we are talking a composite "coating." Cheers