A composite structure having a substrate is protectively covered during fabrication by a barrier layer, such as an intumescent mat or felt, into which a fire resisting agent such as phenolic resin or water, or oil based intumescent coating is infused in-situ before attachment to the substrate and completion of composite structure fabrication. The barrier layer is infused in-situ with the phenolic resin before attachment by adhesive bonding to the substrate or infusion is performed during formation of the underlying substrate layer so as to achieve attachment without adhesive.