Two approaches to determining the open porosity of fiberglass plastic by hydrostatic weighing with and without considering the pore volume and densities of the sample and working fluids, polar ethyl alcohol and non-polar kerosene, have been evaluated. Surface tension and work of adhesion to the fiberglass surface were chosen as evaluation parameters. Surface hydrophobicity, determination of surface tension energy and adhesion work of wetting liquid to the fiberglass plastic surface were evaluated by the edge angle of wetting drops of working fluids. Distilled water was used as a reference for comparison. The evaporation rate and permeation capacity of ethyl alcohol showed significantly higher values based on the measurement of the wetting edge angle. At the same time, wetting of the fiberglass surface was faster when non-polar kerosene was used as the working fluid. The data of the wetted edge angle has been used to determine the adhesion work on the fiberglass surface, taking into account the perfectly smooth surface. According to the data obtained, kerosene demonstrated excellent wetting ability, relatively low volatility (evaporation) with high permeability. It should be kept in mind that when calculating the open porosity of a sample of polymer composite material, the working fluid should be selected based on the wettability, surface polarity of the studied sample, volatility and non-toxicity. At the same time to measure the open porosity it is necessary to keep in mind that the work spent on adhesion with a polar surface will be higher when using polar ethyl alcohol as a wetting liquid.