This study extends our previous work on liquid-phase deposition (LPD) of dense, high-quality, silicon-dioxide (SiO2) films deposited on Si and GaAs substrates from supersaturated, hexafluorosilicic-acid aqueous at near room temperature. Pretreatment to coat the substrate surface with hydroxyl groups was found to be necessary for rapid and high-quality growth. More recent work has extended the range of LPD SiO2 to plastic. The current paper studies optimal LPD pretreatment of a plastic (ARTON) substrate. It is shown that treating ARTON plastic, first, by exposure to oxygen plasma, second, by potassium manganese (KMnO4) etching, and finally, by H2O2 etching, provides the plastic surface with rich OH-radical formation. The resulting SiO2/ARTON film is of good quality and reliability. Deposition rate is up to 659 Å/h, and the refractive index is about 1.44 with growth at 40°C. A growth mechanism for LPD-SiO2 deposition on plastic is proposed.