Polymer composites have a wide range of chemical applications, with strong performance in the paper and cellulose and sugar-alcohol industry due to the chemical resistance such materials offer because of the presence of the polymer matrix. The literature shows that vinyl ester resins offer chemical resistance for these applications, however at a higher price than commercial resins, such as unsaturated polyesters. The aim of this research was to evaluate the chemical resistance of glass fiber reinforced waterproof polyester resin composites (GFPO). Composite was manufactured by hand lay-up and the waterproof polyester specimens were kept immersed in a buffer solution with pH 3 at 70 ??C for 150 days. To evaluate the behavior of waterproof polyester composites, flexural tests were performed and compared with properties of the vinyl ester composites. After the chemical resistance test, the GFPO composite presented a Young???s modulus of 6 GPa with a maximum flexural of 100.6 N. In conclusion, a waterproof polyester resin composite can be a good option for applications in corrosive environments where a vinyl ester resin composite would be applied.