A variable monoenergy slow positron beam (VMSPB) operating at a high vacuum on insulating materials encounters a problem of significant surface charging effect with time. As a result, positronium formation is inhibited, and the positron annihilation radiation counting rate is reduced; these consequently distorted the experimental positron annihilation and results. To solve such problems, a technique of depositing an ultrathin layer of sputtering noble metals on insulators is developed. We report a successful method of sputtering a few atomic layers of platinum (similar to 1 nm) on a polyamide membrane to completely remove the charging effect for VMSPB applications in insulators.