Synthetic silica has a special place to improve the properties of silicone rubber coatings. Good hydrophobicity can be achieved by modifying these nanoparticles and incorporating them into a suitable polymer matrix. Conventional methods of synthesizing silica nanoparticles are expensive and not eco-friendly. Tetraethyl orthosilicate has been used to synthesize silica nanoparticles, although it is not the best source of silica production due to its high cost. One of the cost-effective sources is sodium silicate (water glass). This study used cheaper materials such as water glass, hydrochloric acid, ethyl acetate and cetyl trimethyl ammonium bromide (CTAB) as a surfactant, to synthesize silica in three different methods based on water glass. The phase of the synthesized particles was investigated by X-ray diffraction, and scanning electron microscopy was conducted to determine their particle size and morphology. Surface modification with fluorinated hydrocarbons or silanes is expensive and harmful to the environment and human health. Therefore, the nanoparticles were modified with inexpensive stearic acid, and then they were incorporated into RTV (room temperature vulcanizing) silicone rubber as fillers. The results showed that with increasing silica content from 0 to 3 wt%, the water contact angle of the nanocomposite increased by 19°. The changes in mechanical and electrical properties of RTV with addition of modified synthesized silica nanoparticles were also investigated.