Abstract: To address issues such as poor temperature resistance and susceptibility to flashover in existing side insulation coatings for zinc oxide varistors, this paper investigates a high-insulation side glaze based on Polytetrafluoroethylene (PTFE). This glaze consists of 80%–90% PTFE powder, 5%–10% nano-TiO2, and 6%–12% nano-SiO2 (by weight). It is applied to the varistor side surface through a process involving sand milling, drying, coating, and stepwise curing. The research results indicate that the modified glaze after sand milling achieves an average particle size of 0.3–0.5 μm, significantly reducing coating porosity and enhancing compactness and moisture resistance. Under the optimized curing process (heating rate of 2–5 °C/min, holding at 210–230 °C for 3–5 h), the resulting coating exhibits excellent electrical insulation performance and thermal stability: it withstands 20 repeated charge transfer tests and two high-current impulse tests (4/10 μs, 110 kA) without flashover, and shows no thermal runaway during the operating duty test. Compared with conventional coatings, this PTFE-based side glaze effectively improves the current-carrying capacity and operational reliability of the varistors, providing key technical support for the development of high-performance arresters.