The simplest way of Retr0Briting the plastic is with "40 Volume Creme rinse developer" which is a thickened 12% hydrogen peroxide solution. Although not strong enough to put a rocket into suborbital space (80% is required) it will sclerose skin. Details are available elsewhere, but after you wash off the peroxide, cover the plastic with a clear coat of matte finish to seal the now opened plastic surface from further yellowing.
Oh, keep your Potassium Permanganate away from the 80% peroxide.
