I'm building a petel, and I can't find any advice in the instructions regarding how many layers of fiberglass I should use. In Nick Schade's book, he uses fiberglass on the outside and kevlar on the inside. Since I only have 4 oz fiberglass, does this mean I should use two layers on the inside to achieve the strength of kevlar? I already glassed the outside with a single layer. Any thoughts would be appreciated. Thanks!